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Chapter VI
PLUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIATION.
Brazil, Bahia, gneiss with disjointed metamorphosed dikes.
Strike of foliation.
Rio de Janeiro, gneiss-granite, embedded fragment in, decomposition of.
La Plata, metamorphic and old volcanic rocks of.
S. Ventana.
Claystone porphyry formation of Patagonia; singular metamorphic rocks;
pseudo-dikes.
Falkland Islands, Palaeozoic fossils of.
Tierra del Fuego, clay-slate formation, cretaceous fossils of; cleavage and
foliation; form of land.
Chonos Archipelago, mica-schists, foliation disturbed by granitic axis;
dikes.
Chiloe.
Concepcion, dikes, successive formation of.
Central and Northern Chile.
Concluding remarks on cleavage and foliation.
Their close analogy and similar origin.
Stratification of metamorphic schists.
Foliation of intrusive rocks.
Relation of cleavage and foliation to the lines of tension during
metamorphosis.
The metamorphic and plutonic formations of the several districts visited by
the "Beagle" will be here chiefly treated of, but only such cases as appear
to me new, or of some special interest, will be described in detail; at the
end of the chapter I will sum up all the facts on cleavage and foliation,--
to which I particularly attended.
BAHIA, BRAZIL: latitude 13 degrees south.
The prevailing rock is gneiss, often passing, by the disappearance of the
quartz and mica, and by the feldspar losing its red colour, into a
brilliantly grey primitive greenstone. Not unfrequently quartz and
hornblende are arranged in layers in almost amorphous feldspar. There is
some fine-grained syenitic granite, orbicularly marked by ferruginous
lines, and weathering into vertical, cylindrical holes, almost touching
each other. In the gneiss, concretions of granular feldspar and others of
garnets with mica occur. The gneiss is traversed by numerous dikes composed
of black, finely crystallised, hornblendic rock, containing a little glassy
feldspar and sometimes mica, and varying in thickness from mere threads to
ten feet: these threads, which are often curvilinear, could sometimes be
traced running into the larger dikes. One of these dikes was remarkable
from having been in two or three places laterally disjointed, with unbroken
gneiss interposed between the broken ends, and in one part with a portion
of the gneiss driven, apparently whilst in a softened state, into its side
or wall. In several neighbouring places, the gneiss included angular, well-
defined, sometimes bent, masses of hornblende rock, quite like, except in
being more perfectly crystallised, that forming the dikes, and, at least in
one instance, containing (as determined by Professor Miller) augite as well
as hornblende. In one or two cases these angular masses, though now quite
separate from each other by the solid gneiss, had, from their exact
correspondence in size and shape, evidently once been united; hence I
cannot doubt that most or all of the fragments have been derived from the
breaking up of the dikes, of which we see the first stage in the above-
mentioned laterally disjointed one. The gneiss close to the fragments
generally contained many large crystals of hornblende, which are entirely
absent or rare in other parts: its folia or laminae were gently bent round
the fragments, in the same manner as they sometimes are round concretions.
Hence the gneiss has certainly been softened, its composition modified, and
its folia arranged, subsequently to the breaking up of the dikes, these
latter also having been at the same time bent and softened. (Professor
Hitchcock "Geology of Massachusetts" volume 2 page 673, gives a closely
similar case of a greenstone dike in syenite.)
I must here take the opportunity of premising, that by the term CLEAVAGE I
imply those planes of division which render a rock, appearing to the eye
quite or nearly homogeneous, fissile. By the term FOLIATION, I refer to the
layers or plates of different mineralogical nature of which most
metamorphic schists are composed; there are, also, often included in such
masses, alternating, homogeneous, fissile layers or folia, and in this case
the rock is both foliated and has a cleavage. By STRATIFICATION, as applied
to these formations, I mean those alternate, parallel, large masses of
different composition, which are themselves frequently either foliated or
fissile,--such as the alternating so-called strata of mica-slate, gneiss,
glossy clay-slate, and marble.
The folia of the gneiss within a few miles round Bahia generally strike
irregularly, and are often curvilinear, dipping in all directions at
various angles: but where best defined, they extended most frequently in a
N.E. by N. (or East 50 degrees N.) and S.W. by S. line, corresponding
nearly with the coast-line northwards of the bay. I may add that Mr.
Gardner found in several parts of the province of Ceara, which lies between
four and five hundred miles north of Bahia, gneiss with the folia extending
E. 45 degrees N.; and in Guyana according to Sir R. Schomburgk, the same
rock strikes E. 57 degrees N. Again, Humboldt describes the gneiss-granite
over an immense area in Venezuela and even in Colombia, as striking E. 50
degrees N., and dipping to the N.W. at an angle of fifty degrees. (Gardner
"Geological Section of the British Association" 1840. For Sir R.
Schomburgk's observations see "Geographical Journal" 1842 page 190. See
also Humboldt's discussion on Loxodrism in the "Personal Narrative.") Hence
all the observations hitherto made tend to show that the gneissic rocks
over the whole of this part of the continent have their folia extending
generally within almost a point of the compass of the same direction. (I
landed at only one place north of Bahia, namely, at Pernambuco. I found
there only soft, horizontally stratified matter, formed from disintegrated
granitic rocks, and some yellowish impure limestone, probably of a tertiary
epoch. I have described a most singular natural bar of hard sandstone,
which protects the harbour, in the 19th volume 1841 page 258 of the "London
and Edinburgh Philosophical Magazine."
ABROLHOS ISLETS, Latitude 18 degrees S. off the coast of Brazil.
Although not strictly in place, I do not know where I can more conveniently
describe this little group of small islands. The lowest bed is a sandstone
with ferruginous veins; it weathers into an extraordinary honeycombed mass;
above it there is a dark-coloured argillaceous shale; above this a coarser
sandstone--making a total thickness of about sixty feet; and lastly, above
these sedimentary beds, there is a fine conformable mass of greenstone, in
some parts having a columnar structure. All the strata, as well as the
surface of the land, dip at an angle of about 12 degrees to N. by W. Some
of the islets are composed entirely of the sedimentary, others of the
trappean rocks, generally, however, with the sandstone, cropping out on the
southern shores.)
RIO DE JANEIRO.
This whole district is almost exclusively formed of gneiss, abounding with
garnets, and porphyritic with large crystals, even three and four inches in
length, of orthoclase feldspar: in these crystals mica and garnets are
often enclosed. At the western base of the Corcovado, there is some
ferruginous carious quartz-rock; and in the Tijeuka range, much fine-
grained granite. I observed boulders of greenstone in several places; and
on the islet of Villegagnon, and likewise on the coast some miles
northward, two large trappean dikes. The porphyritic gneiss, or gneiss-
granite as it has been called by Humboldt, is only so far foliated that the
constituent minerals are arranged with a certain degree of regularity, and
may be said to have a "GRAIN," but they are not separated into distinct
folia or laminae. There are, however, several other varieties of gneiss
regularly foliated, and alternating with each other in so-called strata.
The stratification and foliation of the ordinary gneisses, and the
foliation or "grain" of the gneiss-granite, are parallel to each other, and
generally strike within a point of N.E. and S.W. dipping at a high angle
(between 50 and 60 degrees) generally to S.E.: so that here again we meet
with the strike so prevalent over the more northern parts of this
continent. The mountains of gneiss-granite are to a remarkable degree
abruptly conical, which seems caused by the rock tending to exfoliate in
thick, conically concentric layers: these peaks resemble in shape those of
phonolite and other injected rocks on volcanic islands; nor is the grain or
foliation (as we shall afterwards see) any difficulty on the idea of the
gneiss-granite having been an intrusive rather than a metamorphic
formation. The lines of mountains, but not always each separate hill, range
nearly in the same direction with the foliation and so-called
stratification, but rather more easterly.
(FIGURE 22. FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE SAME
ROCK.)
On a bare gently inclined surface of the porphyritic gneiss in Botofogo
Bay, I observed the appearance represented in Figure 22.
A fragment seven yards long and two in width, with angular and distinctly
defined edges, composed of a peculiar variety of gneiss with dark layers of
mica and garnets, is surrounded on all sides by the ordinary gneiss-
granite; both having been dislocated by a granitic vein. The folia in the
fragment and in the surrounding rock strike in the same N.N.E. and S.S.W.
line; but in the fragment they are vertical, whereas in the gneiss-granite
they dip at a small angle, as shown by the arrows, to S.S.E. This fragment,
considering its great size, its solitary position, and its foliated
structure parallel to that of the surrounding rock, is, as far as I know, a
unique case: and I will not attempt any explanation of its origin.
The numerous travellers in this country, have all been greatly surprised at
the depth to which the gneiss and other granitic rocks, as well as the
talcose slates of the interior, have been decomposed. (Spix and Martius
have collected in an Appendix to their "Travels," the largest body of facts
on this subject. See also some remarks by M. Lund in his communications to
the Academy at Copenhagen; and others by M. Gaudichaud in Freycinet
"Voyage.") Near Rio, every mineral except the quartz has been completely
softened, in some places to a depth little less than one hundred feet. (Dr.
Benza describes granitic rock, "Madras Journal of Literature" etc. October
183? page 246), in the Neelgherries, decomposed to a depth of forty feet.)
The minerals retain their positions in folia ranging in the usual
direction; and fractured quartz veins may be traced from the solid rock,
running for some distance into the softened, mottled, highly coloured,
argillaceous mass. It is said that these decomposed rocks abound with gems
of various kinds, often in a fractured state, owing, as some have supposed,
to the collapse of geodes, and that they contain gold and diamonds. At Rio,
it appeared to me that the gneiss had been softened before the excavation
(no doubt by the sea) of the existing, broad, flat-bottomed valleys; for
the depth of decomposition did not appear at all conformable with the
present undulations of the surface. The porphyritic gneiss, where now
exposed to the air, seems to withstand decomposition remarkably well; and I
could see no signs of any tendency to the production of argillaceous masses
like those here described. I was also struck with the fact, that where a
bare surface of this rock sloped into one of the quiet bays, there were no
marks of erosion at the level of the water, and the parts both beneath and
above it preserved a uniform curve. At Bahia, the gneiss rocks are
similarly decomposed, with the upper parts insensibly losing their
foliation, and passing, without any distinct line of separation, into a
bright red argillaceous earth, including partially rounded fragments of
quartz and granite. From this circumstance, and from the rocks appearing to
have suffered decomposition before the excavation of the valleys, I suspect
that here, as at Rio, the decomposition took place under the sea. The
subject appeared to me a curious one, and would probably well repay careful
examination by an able mineralogist.
THE NORTHERN PROVINCES OF LA PLATA.
According to some observations communicated to me by Mr. Fox, the coast
from Rio de Janeiro to the mouth of the Plata seems everywhere to be
granitic, with a few trappean dikes. At Port Alegre, near the boundary of
Brazil, there are porphyries and diorites. (M. Isabelle "Voyage a Buenos
Ayres" page 479.) At the mouth of the Plata, I examined the country for
twenty-five miles west, and for about seventy miles north of Maldonado:
near this town, there is some common gneiss, and much, in all parts of the
country, of a coarse-grained mixture of quartz and reddish feldspar, often,
however, assuming a little dark-green imperfect hornblende, and then
immediately becoming foliated. The abrupt hillocks thus composed, as well
as the highly inclined folia of the common varieties of gneiss, strike
N.N.E. or a little more easterly, and S.S.W. Clay-slate is occasionally met
with, and near the L. del Potrero, there is white marble, rendered fissile
from the presence of hornblende, mica, and asbestus; the cleavage of these
rocks and their stratification, that is the alternating masses thus
composed, strike N.N.E. and S.S.W. like the foliated gneisses, and have an
almost vertical dip. The Sierra Larga, a low range five miles west of
Maldonado, consists of quartzite, often ferruginous, having an arenaceous
feel, and divided into excessively thin, almost vertical laminae or folia
by microscopically minute scales, apparently of mica, and striking in the
usual N.N.E. and S.S.W. direction. The range itself is formed of one
principal line with some subordinate ones; and it extends with remarkable
uniformity far northward (it is said even to the confines of Brazil), in
the same line with the vertically ribboned quartz rock of which it is
composed. The S. de Las Animas is the highest range in the country; I
estimated it at 1,000 feet; it runs north and south, and is formed of
feldspathic porphyry; near its base there is a N.N.W. and S.S.E. ridge of a
conglomerate in a highly porphyritic basis.
Northward of Maldonado, and south of Las Minas, there is an E. and W. hilly
band of country, some miles in width, formed of siliceous clay-slate, with
some quartz, rock, and limestone, having a tortuous irregular cleavage,
generally ranging east and west. E. and S.E. of Las Minas there is a
confused district of imperfect gneiss and laminated quartz, with the hills
ranging in various directions, but with each separate hill generally
running in the same line with the folia of the rocks of which it is
composed: this confusion appears to have been caused by the intersection of
the [E. and W.] and [N.N.E. and S.S.W.] strikes. Northward of Las Minas,
the more regular northerly ranges predominate: from this place to near
Polanco, we meet with the coarse-grained mixture of quartz and feldspar,
often with the imperfect hornblende, and then becoming foliated in a N. and
S. line--with imperfect clay-slate, including laminae of red crystallised
feldspar--with white or black marble, sometimes containing asbestus and
crystals of gypsum--with quartz-rock--with syenite--and lastly, with much
granite. The marble and granite alternate repeatedly in apparently vertical
masses: some miles northward of the Polanco, a wide district is said to be
entirely composed of marble. It is remarkable, how rare mica is in the
whole range of country north and westward of Maldonado. Throughout this
district, the cleavage of the clay-slate and marble--the foliation of the
gneiss and the quartz--the stratification or alternating masses of these
several rocks--and the range of the hills, all coincide in direction; and
although the country is only hilly, the planes of division are almost
everywhere very highly inclined or vertical.
Some ancient submarine volcanic rocks are worth mentioning, from their
rarity on this eastern side of the continent. In the valley of the Tapas
(fifty or sixty miles N. of Maldonado) there is a tract three or four miles
in length, composed of various trappean rocks with glassy feldspar--of
apparently metamorphosed grit-stones--of purplish amygdaloids with large
kernels of carbonate of lime (Near the Pan de Azucar there is some greenish
porphyry, in one place amygdaloidal with agate.)--and much of a harshish
rock with glassy feldspar intermediate in character between claystone
porphyry and trachyte. This latter rock was in one spot remarkable from
being full of drusy cavities, lined with quartz crystals, and arranged in
planes, dipping at an angle of 50 degrees to the east, and striking
parallel to the foliation of an adjoining hill composed of the common
mixture of quartz, feldspar, and imperfect hornblende: this fact perhaps
indicates that these volcanic rocks have been metamorphosed, and their
constituent parts rearranged, at the same time and according to the same
laws, with the granitic and metamorphic formations of this whole region. In
the valley of the Marmaraya, a few miles south of the Tapas, a band of
trappean and amygdaloidal rock is interposed between a hill of granite and
an extensive surrounding formation of red conglomerate, which (like that at
the foot of the S. Animas) has its basis porphyritic with crystals of
feldspar, and which hence has certainly suffered metamorphosis.
MONTE VIDEO.
The rocks here consist of several varieties of gneiss, with the feldspar
often yellowish, granular and imperfectly crystallised, alternating with,
and passing insensibly into, beds, from a few yards to nearly a mile in
thickness, of fine or coarse grained, dark-green hornblendic slate; this
again often passing into chloritic schist. These passages seem chiefly due
to changes in the mica, and its replacement by other minerals. At Rat
Island I examined a mass of chloritic schist, only a few yards square,
irregularly surrounded on all sides by the gneiss, and intricately
penetrated by many curvilinear veins of quartz, which gradually BLEND into
the gneiss: the cleavage of the chloritic schist and the foliation of the
gneiss were exactly parallel. Eastward of the city there is much fine-
grained, dark-coloured gneiss, almost assuming the character of hornblende-
slate, which alternates in thin laminae with laminae of quartz, the whole
mass being transversely intersected by numerous large veins of quartz: I
particularly observed that these veins were absolutely continuous with the
alternating laminae of quartz. In this case and at Rat Island, the passage
of the gneiss into imperfect hornblendic or into chloritic slate, seemed to
be connected with the segregation of the veins of quartz. (Mr. Greenough
page 78 "Critical Examination" etc., observes that quartz in mica-slate
sometimes appears in beds and sometimes in veins. Von Buch also in his
"Travels in Norway" page 236, remarks on alternating laminae of quartz and
hornblende-slate replacing mica-schist.)
The Mount, a hill believed to be 450 feet in height, from which the place
takes its name, is much the highest land in this neighbourhood: it consists
of hornblendic slate, which (except on the eastern and disturbed base) has
an east and west nearly vertical cleavage; the longer axis of the hill also
ranges in this same line. Near the summit the hornblende-slate gradually
becomes more and more coarsely crystallised, and less plainly laminated,
until it passes into a heavy, sonorous greenstone, with a slaty conchoidal
fracture; the laminae on the north and south sides near the summit dip
inwards, as if this upper part had expanded or bulged outwards. This
greenstone must, I conceive, be considered as metamorphosed hornblende-
slate. The Cerrito, the next highest, but much less elevated point, is
almost similarly composed. In the more western parts of the province,
besides gneiss, there is quartz-rock, syenite, and granite; and at Colla, I
heard of marble.
Near M. Video, the space which I more accurately examined was about fifteen
miles in an east and west line, and here I found the foliation of the
gneiss and the cleavage of the slates generally well developed, and
extending parallel to the alternating strata composed of the gneiss,
hornblendic and chloritic schists. These planes of division all range
within one point of east and west, frequently east by south and west by
north; their dip is generally almost vertical, and scarcely anywhere under
45 degrees: this fact, considering how slightly undulatory the surface of
the country is, deserves attention. Westward of M. Video, towards the
Uruguay, wherever the gneiss is exposed, the highly inclined folia are seen
striking in the same direction; I must except one spot where the strike was
N.W. by W. The little Sierra de S. Juan, formed of gneiss and laminated
quartz, must also be excepted, for it ranges between [N. to N.E.] and [S.
to S.W.] and seems to belong to the same system with the hills in the
Maldonado district. Finally, we have seen that, for many miles northward of
Maldonado and for twenty-five miles westward of it, as far as the S. de las
Animas, the foliation, cleavage, so-called stratification and lines of
hills, all range N.N.E. and S.S.W., which is nearly coincident with the
adjoining coast of the Atlantic. Westward of the S. de las Animas, as far
as even the Uruguay, the foliation, cleavage, and stratification (but not
lines of hills, for there are no defined ones) all range about E. by S. and
W. by N., which is nearly coincident with the direction of the northern
shore of the Plata; in the confused country near Las Minas, where these two
great systems appear to intersect each other, the cleavage, foliation, and
stratification run in various directions, but generally coincide with the
line of each separate hill.
SOUTHERN LA PLATA.
The first ridge, south of the Plata, which projects through the Pampean
formation, is the Sierra Tapalguen and Vulcan, situated 200 miles southward
of the district just described. This ridge is only a few hundred feet in
height, and runs from C. Corrientes in a W.N.W. line for at least 150 miles
into the interior: at Tapalguen, it is composed of unstratified granular
quartz, remarkable from forming tabular masses and small plains, surrounded
by precipitous cliffs: other parts of the range are said to consist of
granite: and marble is found at the S. Tinta. It appears from M.
Parchappe's observations, that at Tandil there is a range of quartzose
gneiss, very like the rocks of the S. Larga near Maldonado, running in the
same N.N.E. and S.S.W. direction; so that the framework of the country here
is very similar to that on the northern shore of the Plata. (M. d'Orbigny's
"Voyage" Part. Geolog. page 46. I have given a short account of the
peculiar forms of the quartz hills of Tapalguen, so unusual in a
metamorphic formation, in my "Journal of Researches" 2nd edition page 116.)
The Sierra Guitru-gueyu is situated sixty miles south of the S. Tapalguen:
it consists of numerous parallel, sometimes blended together ridges, about
twenty-three miles in width, and five hundred feet in height above the
plain, and extending in a N.W. and S.E. direction. Skirting round the
extreme S.E. termination, I ascended only a few points, which were composed
of a fine-grained gneiss, almost composed of feldspar with a little mica,
and passing in the upper parts of the hills into a rather compact purplish
clay-slate. The cleavage was nearly vertical, striking in a N.W. by W. and
S.E. by E. line, nearly, though not quite, coincident with the direction of
the parallel ridges.
The Sierra Ventana lies close south of that of Guitru-gueyu; it is
remarkable from attaining a height, very unusual on this side of the
continent, of 3,340 feet. It consists up to its summit, of quartz,
generally pure and white, but sometimes reddish, and divided into thick
laminae or strata: in one part there is a little glossy clay-slate with a
tortuous cleavage. The thick layers of quartz strike in a W. 30 degrees N.
line, dipping southerly at an angle of 45 degrees and upwards. The
principal line of mountains, with some quite subordinate parallel ridges,
range about W. 45 degrees N.: but at their S.E. termination, only W. 25
degrees N. This Sierra is said to extend between twenty and thirty leagues
into the interior.
PATAGONIA.
With the exception perhaps of the hill of S. Antonio (600 feet high) in the
Gulf of S. Matias, which has never been visited by a geologist, crystalline
rocks are not met with on the coast of Patagonia for a space of 380 miles
south of the S. Ventana. At this point (latitude 43 degrees 50 minutes), at
Points Union and Tombo, plutonic rocks are said to appear, and are found,
at rather wide intervals, beneath the Patagonian tertiary formation for a
space of about three hundred miles southward, to near Bird Island, in
latitude 48 degrees 56 minutes. Judging from specimens kindly collected for
me by Mr. Stokes, the prevailing rock at Ports St. Elena, Camerones,
Malaspina, and as far south as the Paps of Pineda, is a purplish-pink or
brownish claystone porphyry, sometimes laminated, sometimes slightly
vesicular, with crystals of opaque feldspar and with a few grains of
quartz; hence these porphyries resemble those immediately to be described
at Port Desire, and likewise a series which I have seen from P. Alegre on
the southern confines of Brazil. This porphyritic formation further
resembles in a singularly close manner the lowest stratified formation of
the Cordillera of Chile, which, as we shall hereafter see, has a vast
range, and attains a great thickness. At the bottom of the Gulf of St.
George, only tertiary deposits appear to be present. At Cape Blanco, there
is quartz rock, very like that of the Falkland Islands, and some hard, blue
siliceous clay-slate.
At Port Desire there is an extensive formation of the claystone porphyry,
stretching at least twenty-five miles into the interior: it has been
denuded and deeply worn into gullies before being covered up by the
tertiary deposits, through which it here and there projects in hills; those
north of the bay being 440 feet in height. The strata have in several
places been tilted at small angles, generally either to N.N.W. or S.S.E. By
gradual passages and alternations, the porphyries change incessantly in
nature. I will describe only some of the principal mineralogical changes,
which are highly instructive, and which I carefully examined. The
prevailing rock has a compact purplish base, with crystals of earthy or
opaque feldspar, and often with grains of quartz. There are other
varieties, with an almost truly trachytic base, full of little angular
vesicles and crystals of glassy feldspar; and there are beds of black
perfect pitchstone, as well as of a concretionary imperfect variety. On a
casual inspection, the whole series would be thought to be of the same
plutonic or volcanic nature with the trachytic varieties and pitchstone;
but this is far from being the case, as much of the porphyry is certainly
of metamorphic origin. Besides the true porphyries, there are many beds of
earthy, quite white or yellowish, friable, easily fusible matter,
resembling chalk, which under the microscope is seen to consist of minute
broken crystals, and which, as remarked in a former chapter, singularly
resembles the upper tufaceous beds of the Patagonian tertiary formation.
This earthy substance often becomes coarser, and contains minute rounded
fragments of porphyries and rounded grains of quartz, and in one case so
many of the latter as to resemble a common sandstone. These beds are
sometimes marked with true lines of aqueous deposition, separating
particles of different degrees of coarseness; in other cases there are
parallel ferruginous lines not of true deposition, as shown by the
arrangement of the particles, though singularly resembling them. The more
indurated varieties often include many small and some larger angular
cavities, which appear due to the removal of earthy matter: some varieties
contain mica. All these earthy and generally white stones insensibly pass
into more indurated sonorous varieties, breaking with a conchoidal
fracture, yet of small specific gravity; many of these latter varieties
assume a pale purple tint, being singularly banded and veined with
different shades, and often become plainly porphyritic with crystals of
feldspar. The formation of these crystals could be most clearly traced by
minute angular and often partially hollow patches of earthy matter, first
assuming a FIBROUS STRUCTURE, then passing into opaque imperfectly shaped
crystals, and lastly, into perfect glassy crystals. When these crystals
have appeared, and when the basis has become compact, the rock in many
places could not be distinguished from a true claystone porphyry without a
trace of mechanical structure.
In some parts, these earthy or tufaceous beds pass into jaspery and into
beautifully mottled and banded porcelain rocks, which break into splinters,
translucent at their edges, hard enough to scratch glass, and fusible into
white transparent beads: grains of quartz included in the porcelainous
varieties can be seen melting into the surrounding paste. In other parts,
the earthy or tufaceous beds either insensibly pass into, or alternate
with, breccias composed of large and small fragments of various purplish
porphyries, with the matrix generally porphyritic: these breccias, though
their subaqueous origin is in many places shown both by the arrangement of
their smaller particles and by an oblique or current lamination, also pass
into porphyries, in which every trace of mechanical origin and
stratification has been obliterated.
Some highly porphyritic though coarse-grained masses, evidently of
sedimentary origin, and divided into thin layers, differing from each other
chiefly in the number of embedded grains of quartz, interested me much from
the peculiar manner in which here and there some of the layers terminated
in abrupt points, quite unlike those produced by a layer of sediment
naturally thinning out, and apparently the result of a subsequent process
of metamorphic aggregation. In another common variety of a finer texture,
the aggregating process had gone further, for the whole mass consisted of
quite short, parallel, often slightly curved layers or patches, of whitish
or reddish finely granulo-crystalline feldspathic matter, generally
terminating at both ends in blunt points; these layers or patches further
tended to pass into wedge or almond-shaped little masses, and these finally
into true crystals of feldspar, with their centres often slightly drusy.
The series was so perfect that I could not doubt that these large crystals,
which had their longer axes placed parallel to each other, had primarily
originated in the metamorphosis and aggregation of alternating layers of
tuff; and hence their parallel position must be attributed (unexpected
though the conclusion may be), not to laws of chemical action, but to the
original planes of deposition. I am tempted briefly to describe three other
singular allied varieties of rock; the first without examination would have
passed for a stratified porphyritic breccia, but all the included angular
fragments consisted of a border of pinkish crystalline feldspathic matter,
surrounding a dark translucent siliceous centre, in which grains of quartz
not quite blended into the paste could be distinguished: this uniformity in
the nature of the fragments shows that they are not of mechanical, but of
concretionary origin, having resulted perhaps from the self-breaking up and
aggregation of layers of indurated tuff containing numerous grains of
quartz,--into which, indeed, the whole mass in one part passed. The second
variety is a reddish non-porphyritic claystone, quite full of spherical
cavities, about half an inch in diameter, each lined with a collapsed crust
formed of crystals of quartz. The third variety also consists of a pale
purple non-porphyritic claystone, almost wholly formed of concretionary
balls, obscurely arranged in layers, of a less compact and paler coloured
claystone; each ball being on one side partly hollow and lined with
crystals of quartz.
PSEUDO-DIKES.
Some miles up the harbour, in a line of cliffs formed of slightly
metamorphosed tufaceous and porphyritic claystone beds, I observed three
vertical dikes, so closely resembling in general appearance ordinary
volcanic dikes, that I did not doubt, until closely examining their
composition, that they had been injected from below. The first is straight,
with parallel sides, and about four feet wide; it consists of whitish,
indurated tufaceous matter, precisely like some of the beds intersected by
it. The second dike is more remarkable; it is slightly tortuous, about
eighteen inches thick, and can be traced for a considerable distance along
the beach; it is of a purplish-red or brown colour, and is formed chiefly
of ROUNDED grains of quartz, with broken crystals of earthy feldspar,
scales of black mica, and minute fragments of claystone porphyry, all
firmly united together in a hard sparing base. The structure of this dike
shows obviously that it is of mechanical and sedimentary origin; yet it
thinned out upwards, and did not cut through the uppermost strata in the
cliffs. This fact at first appears to indicate that the matter could not
have been washed in from above (Upfilled fissures are known to occur both
in volcanic and in ordinary sedimentary formations. At the Galapagos
Archipelago "Volcanic Islands" etc., there are some striking examples of
pseudo-dikes composed of hard tuff.); but if we reflect on the suction
which would result from a deep-seated fissure being formed, we may admit
that if the fissure were in any part open to the surface, mud and water
might well be drawn into it along its whole course. The third dike
consisted of a hard, rough, white rock, almost composed of broken crystals
of glassy feldspar, with numerous scales of black mica, cemented in a
scanty base; there was little in the appearance of this rock, to preclude
the idea of its having been a true injected feldspathic dike. The matter
composing these three pseudo-dikes, especially the second one, appears to
have suffered, like the surrounding strata, a certain degree of metamorphic
action; and this has much aided the deceptive appearance. At Bahia, in
Brazil, we have seen that a true injected hornblendic dike, not only has
suffered metamorphosis, but has been dislocated and even diffused in the
surrounding gneiss, under the form of separate crystals and of fragments.
FALKLAND ISLANDS.
I have described these islands in a paper published in the third volume of
the "Geological Journal." The mountain-ridges consist of quartz, and the
lower country of clay-slate and sandstone, the latter containing Palaeozoic
fossils. These fossils have been separately described by Messrs. Morris and
Sharpe: some of them resemble Silurian, and others Devonian forms. In the
eastern part of the group the several parallel ridges of quartz extend in a
west and east line; but further westward the line becomes W.N.W. and
E.S.E., and even still more northerly. The cleavage-planes of the clay-
slate are highly inclined, generally at an angle of above 50 degrees, and
often vertical; they strike almost invariably in the same direction with
the quartz ranges. The outline of the indented shores of the two main
islands, and the relative positions of the smaller islets, accord with the
strike both of the main axes of elevation and of the cleavage of the clay-
slate.
TIERRA DEL FUEGO.
My notes on the geology of this country are copious, but as they are
unimportant, and as fossils were found only in one district, a brief sketch
will be here sufficient. The east coast from the S. of Magellan (where the
boulder formation is largely developed) to St. Polycarp's Bay is formed of
horizontal tertiary strata, bounded some way towards the interior by a
broad mountainous band of clay-slate. This great clay-slate formation
extends from St. Le Maire westward for 140 miles, along both sides of the
Beagle Channel to near its bifurcation. South of this channel, it forms all
Navarin Island, and the eastern half of Hoste Island and of Hardy
Peninsula; north of the Beagle Channel it extends in a north-west line on
both sides of Admiralty Sound to Brunswick Peninsula in the St. of
Magellan, and I have reason to believe, stretches far up the eastern side
of the Cordillera. The western and broken side of Tierra del Fuego towards
the Pacific is formed of metamorphic schists, granite and various trappean
rocks: the line of separation between the crystalline and clay-slate
formations can generally be distinguished, as remarked by Captain King, by
the parallelism in the clay-slate districts of the shores and channels,
ranging in a line between [W. 20 degrees to 40 degrees N.] and [E. 20
degrees to 40 degrees S.]. ("Geographical Journal" volume 1 page 155.)
The clay-slate is generally fissile, sometimes siliceous or ferruginous,
with veins of quartz and calcareous spar; it often assumes, especially on
the loftier mountains, an altered feldspathic character, passing into
feldspathic porphyry: occasionally it is associated with breccia and
grauwacke. At Good Success Bay, there is a little intercalated black
crystalline limestone. At Port Famine much of the clay-slate is calcareous,
and passes either into a mudstone or into grauwacke, including odd-shaped
concretions of dark argillaceous limestone. Here alone, on the shore a few
miles north of Port Famine, and on the summit of Mount Tarn (2,600 feet
high), I found organic remains; they consist of:--
1. Ancyloceras simplex, d'Orbigny "Pal Franc" Mount Tarn.
2. Fusus (in imperfect state), d'Orbigny "Pal Franc" Mount Tarn.
3. Natica, d'Orbigny "Pal Franc" Mount Tarn.
4. Pentacrimus, d'Orbigny "Pal Franc" Mount Tarn.
5. Lucina excentrica, G.B. Sowerby, Port Famine.
6. Venus (in imperfect state), G.B. Sowerby, Port Famine.
7. Turbinolia (?), G.B. Sowerby, Port Famine.
8. Hamites elatior, G.B. Sowerby, Port Famine.
M. d'Orbigny states that MM. Hombron and Grange found in this neighbourhood
an Ancyloceras, perhaps A. simplex, an Ammonite, a Plicatula and Modiola.
("Voyage" Part Geolog. page 242.) M. d'Orbigny believes from the general
character of these fossils, and from the Ancyloceras being identical (as
far as its imperfect condition allows of comparison) with the A. simplex of
Europe, that the formation belongs to an early stage of the Cretaceous
system. Professor E. Forbes, judging only from my specimens, concurs in the
probability of this conclusion. The Hamites elatior of the above list, of
which a description has been given by Mr. Sowerby, and which is remarkable
from its large size, has not been seen either by M. d'Orbigny or Professor
E. Forbes, as, since my return to England, the specimens have been lost.
The great clay-slate formation of Tierra del Fuego being cretaceous, is
certainly a very interesting fact,--whether we consider the appearance of
the country, which, without the evidence afforded by the fossils, would
form the analogy of most known districts, probably have been considered as
belonging to the Palaeozoic series,--or whether we view it as showing that
the age of this terminal portion of the great axis of South America, is the
same (as will hereafter be seen) with the Cordillera of Chile and Peru.
The clay-slate in many parts of Tierra del Fuego, is broken by dikes and by
great masses of greenstone, often highly hornblendic (In a greenstone-dike
in the Magdalen Channel, the feldspar cleaved with the angle of albite.
This dike was crossed, as well as the surrounding slate, by a large vein of
quartz, a circumstance of unusual occurrence.): almost all the small islets
within the clay-slate districts are thus composed. The slate near the dikes
generally becomes paler-coloured, harder, less fissile, of a feldspathic
nature, and passes into a porphyry or greenstone: in one case, however, it
became more fissile, of a red colour, and contained minute scales of mica,
which were absent in the unaltered rock. On the east side of Ponsonby Sound
some dikes composed of a pale sonorous feldspathic rock, porphyritic with a
little feldspar, were remarkable from their number,--there being within the
space of a mile at least one hundred,--from their nearly equalling in bulk
the intermediate slate,--and more especially from the excessive fineness
(like the finest inlaid carpentry) and perfect parallelism of their
junctions with the almost vertical laminae of clay-slate. I was unable to
persuade myself that these great parallel masses had been injected, until I
found one dike which abruptly thinned out to half its thickness, and had
one of its walls jagged, with fragments of the slate embedded in it.
In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary,
becomes much altered and feldspathic. Thus on Wollaston Island slate and
grauwacke can be distinctly traced passing into feldspathic rocks and
greenstones, including iron pyrites and epidote, but still retaining traces
of cleavage with the usual strike and dip. One such metamorphosed mass was
traversed by large vein-like masses of a beautiful mixture (as ascertained
by Professor Miller) of green epidote, garnets, and white calcareous spar.
On the northern point of this same island, there were various ancient
submarine volcanic rocks, consisting of amygdaloids with dark bole and
agate,--of basalt with decomposed olivine--of compact lava with glassy
feldspar,--and of a coarse conglomerate of red scoriae, parts being
amygdaloidal with carbonate of lime. The southern part of Wollaston Island
and the whole of Hermite and Horn Islands, seem formed of cones of
greenstone; the outlying islets of Il Defenso and D. Raminez are said to
consist of porphyritic lava. (Determined by Professor Jameson. Weddell's
"Voyage" page 169.) In crossing Hardy Peninsula, the slate still retaining
traces of its usual cleavage, passes into columnar feldspathic rocks, which
are succeeded by an irregular tract of trappean and basaltic rocks,
containing glassy feldspar and much iron pyrites: there is, also, some
harsh red claystone porphyry, and an almost true trachyte, with needles of
hornblende, and in one spot a curious slaty rock divided into quadrangular
columns, having a base almost like trachyte, with drusy cavities lined by
crystals, too imperfect, according to Professor Miller, to be measured, but
resembling Zeagonite. (See Mr. Brooke's Paper in the "London Philosophical
Magazine" volume 10. This mineral occurs in an ancient volcanic rock near
Rome.) In the midst of these singular rocks, no doubt of ancient submarine
volcanic origin, a high hill of feldspathic clay-slate projected, retaining
its usual cleavage. Near this point, there was a small hillock, having the
aspect of granite, but formed of white albite, brilliant crystals of
hornblende (both ascertained by the reflecting goniometer) and mica; but
with no quartz. No recent volcanic district has been observed in any part
of Tierra del Fuego.
Five miles west of the bifurcation of the Beagle Channel, the slate-
formation, instead of becoming, as in the more southern parts of Tierra del
Fuego, feldspathic, and associated with trappean or old volcanic rocks,
passes by alternations into a great underlying mass of fine gneiss and
glossy clay-slate, which at no great distance is succeeded by a grand
formation of mica-slate containing garnets. The folia of these metamorphic
schists strike parallel to the cleavage-planes of the clay-slate, which
have a very uniform direction over the whole of this part of the country:
the folia, however, are undulatory and tortuous, whilst the cleavage-
laminae of the slate are straight. These schists compose the chief
mountain-chain of Southern Tierra del Fuego, ranging along the north side
of the northern arm of the Beagle Channel, in a short W.N.W. and E.S.E.
line, with two points (Mounts Sarmiento and Darwin) rising to heights of
6,800 and 6,900 feet. On the south-western side of this northern arm of the
Beagle Channel, the clay-slate is seen with its STRATA dipping from the
great chain, so that the metamorphic schists here form a ridge bordered on
each side by clay-slate. Further north, however, to the west of this great
range, there is no clay-slate, but only gneiss, mica, and hornblendic
slates, resting on great barren hills of true granite, and forming a tract
about sixty miles in width. Again, westward of these rocks, the outermost
islands are of trappean formation, which, from information obtained during
the voyages of the "Adventure" and "Beagle," seem, together with granite,
chiefly to prevail along the western coast as far north as the entrance of
the St. of Magellan (See the Paper by Captain King in the "Geographical
Journal"; also a Letter to Dr. Fitton in "Geological Proceedings" volume 1
page 29; also some observations by Captain Fitzroy "Voyages" volume 1 page
375. I am indebted also to Mr. Lyell for a series of specimens collected by
Lieutenant Graves.): a little more inland, on the eastern side of Clarence
Island and S. Desolation, granite, greenstone, mica-slate, and gneiss
appear to predominate. I am tempted to believe, that where the clay-slate
has been metamorphosed at great depths beneath the surface, gneiss, mica-
slate, and other allied rocks have been formed, but where the action has
taken place nearer the surface, feldspathic porphyries, greenstones, etc.,
have resulted, often accompanied by submarine volcanic eruptions.
Only one other rock, met with in both arms of the Beagle Channel, deserves
any notice, namely a granulo-crystalline mixture of white albite, black
hornblende (ascertained by measurement of the crystals, and confirmed by
Professor Miller), and more or less of brown mica, but without any quartz.
This rock occurs in large masses, closely resembling in external form
granite or syenite: in the southern arm of the Channel, one such mass
underlies the mica-slate, on which clay-slate was superimposed: this
peculiar plutonic rock which, as we have seen, occurs also in Hardy
Peninsula, is interesting, from its perfect similarity with that (hereafter
often to be referred to under the name of andesite) forming the great
injected axes of the Cordillera of Chile.
The stratification of the clay-slate is generally very obscure, whereas the
cleavage is remarkably well defined: to begin with the extreme eastern
parts of Tierra del Fuego; the cleavage-planes near the St. of Le Maire
strike either W. and E. or W.S.W. and E.N.E., and are highly inclined; the
form of the land, including Staten Island, indicates that the axes of
elevation have run in this same line, though I was unable to distinguish
the planes of stratification. Proceeding westward, I accurately examined
the cleavage of the clay-slate on the northern, eastern, and western sides
(thirty-five miles apart) of Navarin Island, and everywhere found the
laminae ranging with extreme regularity, W.N.W. and E.S.E., seldom varying
more than one point of the compass from this direction. (The clay-slate in
this island was in many places crossed by parallel smooth joints. Out of
five cases, the angle of intersection between the strike of these joints
and that of the cleavage-laminae was in two cases 45 degrees and in two
others 79 degrees.) Both on the east and west coasts, I crossed at right
angles the cleavage-planes for a space of about eight miles, and found them
dipping at an angle of between 45 degrees and 90 degrees, generally to
S.S.W., sometimes to N.N.E., and often quite vertically. The S.S.W. dip was
occasionally succeeded abruptly by a N.N.E. dip, and this by a vertical
cleavage, or again by the S.S.W. dip; as in a lofty cliff on the eastern
end of the island the laminae of slate were seen to be folded into very
large steep curves, ranging in the usual W.N.W. line, I suspect that the
varying and opposite dips may possibly be accounted for by the cleavage-
laminae, though to the eye appearing straight, being parts of large abrupt
curves, with their summits cut off and worn down.
In several places I was particularly struck with the fact, that the fine
laminae of the clay-slate, where cutting straight through the bands of
stratification, and therefore indisputably true cleavage-planes, differed
slightly in their greyish and greenish tints of colour, in compactness, and
in some of the laminae having a rather more jaspery appearance than others.
I have not seen this fact recorded, and it appears to me important, for it
shows that the same cause which has produced the highly fissile structure,
has altered in a slight degree the mineralogical character of the rock in
the same planes. The bands of stratification, just alluded to, can be
distinguished in many places, especially in Navarin Island, but only on the
weathered surfaces of the slate; they consist of slightly undulatory zones
of different shades of colour and of thicknesses, and resemble the marks
(more closely than anything else to which I can compare them) left on the
inside of a vessel by the draining away of some dirty slightly agitated
liquid: no difference in composition, corresponding with these zones, could
be seen in freshly fractured surfaces. In the more level parts of Navarin
Island, these bands of stratification were nearly horizontal; but on the
flanks of the mountains they were inclined from them, but in no instance
that I saw at a very high angle. There can, I think, be no doubt that these
zones, which appear only on the weathered surfaces, are the last vestiges
of the original planes of stratification, now almost obliterated by the
highly fissile and altered structure which the mass has assumed.
The clay-slate cleaves in the same W.N.W. and E.S.E. direction, as on
Navarin Island, on both sides of the Beagle Channel, on the eastern side of
Hoste Island, on the N.E. side of Hardy Peninsula, and on the northern
point of Wollaston Island; although in these two latter localities the
cleavage has been much obscured by the metamorphosed and feldspathic
condition of the slate. Within the area of these several islands, including
Navarin Island, the direction of the stratification and of the mountain-
chains is very obscure; though the mountains in several places appeared to
range in the same W.N.W. line with the cleavage: the outline of the coast,
however, does not correspond with this line. Near the bifurcation of the
Beagle Channel, where the underlying metamorphic schists are first seen,
they are foliated (with some irregularities), in this same W.N.W. line, and
parallel, as before stated, to the main mountain-axis of this part of the
country. Westward of this main range, the metamorphic schists are foliated,
though less plainly, in the same direction, which is likewise common to the
zone of old erupted trappean rocks, forming the outermost islets. Hence the
area, over which the cleavage of the slate and the foliation of the
metamorphic schists extends with an average W.N.W. and E.S.E. strike, is
about forty miles in a north and south line, and ninety miles in an east
and west line.
Further northward, near Port Famine, the stratification of the clay-slate
and of the associated rocks, is well defined, and there alone the cleavage
and strata-planes are parallel. A little north of this port there is an
anticlinal axis ranging N.W. (or a little more westerly) and S.E.: south of
the port, as far as Admiralty Sound and Gabriel Channel, the outline of the
land clearly indicates the existence of several lines of elevation in this
same N.W. direction, which, I may add, is so uniform in the western half of
the St. of Magellan, that, as Captain King has remarked, "a parallel ruler
placed on the map upon the projecting points of the south shore, and
extended across the strait, will also touch the headlands on the opposite
coast." ("Geographical Journal" volume 1 page 170.) It would appear, from
Captain King's observations, that over all this area the cleavage extends
in the same line. Deep-water channels, however, in all parts of Tierra del
Fuego have burst through the trammels both of stratification and cleavage;
most of them may have been formed during the elevation of the land by long-
continued erosion, but others, for instance the Beagle Channel, which
stretches like a narrow canal for 120 miles obliquely through the
mountains, can hardly have thus originated.
Finally, we have seen that in the extreme eastern point of Tierra del
Fuego, the cleavage and coast-lines extend W. and E. and even W.S.W. and
E.N.E.: over a large area westward, the cleavage, the main range of
mountains, and some subordinate ranges, but not the outlines of the coast,
strike W.N.W., and E.S.E.: in the central and western parts of the St. of
Magellan, the stratification, the mountain-ranges, the outlines of the
coast, and the cleavage all strike nearly N.W. and S.E. North of the
strait, the outline of the coast, and the mountains on the mainland, run
nearly north and south. Hence we see, at this southern point of the
continent, how gradually the Cordillera bend, from their north and south
course of so many thousand miles in length, into an E. and even E.N.E.
direction.
WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE.
The first place at which we landed north of the St. of Magellan was near
Cape Tres Montes, in latitude 47 degrees S. Between this point and the
Northern Chonos Islands, a distance of 200 miles, the "Beagle" visited
several points, and specimens were collected for me from the intermediate
spaces by Lieutenant Stokes. The predominant rock is mica-slate, with thick
folia of quartz, very frequently alternating with and passing into a
chloritic, or into a black, glossy, often striated, slightly anthracitic
schist, which soils paper, and becomes white under a great heat, and then
fuses. Thin layers of feldspar, swelling at intervals into well
crystallised kernels, are sometimes included in these black schists; and I
observed one mass of the ordinary black variety insensibly lose its fissile
structure, and pass into a singular mixture of chlorite, epidote, feldspar,
and mica. Great veins of quartz are numerous in the mica-schists; wherever
these occur the folia are much convoluted. In the southern part of the
Peninsula of Tres Montes, a compact altered feldspathic rock with crystals
of feldspar and grains of quartz is the commonest variety; this rock
exhibits occasionally traces of an original brecciated structure, and often
presents (like the altered state of Tierra del Fuego) traces of cleavage-
planes, which strike in the same direction with the folia of mica-schist
further northward. (The peculiar, abruptly conical form of the hills in
this neighbourhood, would have led any one at first to have supposed that
they had been formed of injected or intrusive rocks. At Inchemo Island, a
similar rock gradually becomes granulo-crystalline and acquires scales of
mica; and this variety at S. Estevan becomes highly laminated, and though
still exhibiting some rounded grains of quartz, passes into the black,
glossy, slightly anthracitic schist, which, as we have seen, repeatedly
alternates with and passes into the micaceous and chloritic schists. Hence
all the rocks on this line of coast belong to one series, and insensibly
vary from an altered feldspathic clay-slate into largely foliated, true
mica-schist.
The cleavage of the homogeneous schists, the foliation of those composed of
more or less distinct minerals in layers, and the planes of alternation of
the different varieties or so-called stratification, are all parallel, and
preserve over this 200 miles of coast a remarkable degree of uniformity in
direction. At the northern end of the group, at Low's Harbour, the well-
defined folia of mica-schist everywhere ranged within eight degrees (or
less than one point of the compass) of N. 19 degrees W. and S. 19 degrees
E.; and even the point of dip varied very little, being always directed to
the west and generally at an angle of forty degrees; I should mention that
I had here good opportunities of observation, for I followed the naked rock
on the beach, transversely to the strike, for a distance of four miles and
a half, and all the way attended to the dip. Along the outer islands for
100 miles south of Low's Harbour, Lieutenant Stokes, during his boat-
survey, kindly observed for me the strike of the foliation, and he assures
me that it was invariably northerly, and the dip with one single exception
to the west. Further south at Vallenar Bay, the strike was almost
universally N. 25 degrees W. and the dip, generally at an angle of about 40
degrees to W. 25 degrees S., but in some places almost vertical. Still
farther south, in the neighbourhood of the harbours of Anna Pink, S.
Estevan and S. Andres, and (judging from a distance) along the southern
part of Tres Montes, the foliation and cleavage extended in a line between
[N. 11 degrees to 22 degrees W.] and [S. 11 degrees to 22 degrees E.]; and
the planes dipped generally westerly, but often easterly, at angles varying
from a gentle inclination to vertical. At A. Pink's Harbour, where the
schists generally dipped easterly, wherever the angle became very high, the
strike changed from N. 11 degrees W. to even as much as N. 45 degrees W.:
in an analogous manner at Vallenar Bay, where the dip was westerly (viz. on
an average directed to W. 25 degrees S.), as soon as the angle became very
high, the planes struck in a line more than 25 degrees west of north. The
average result from all the observations on this 200 miles of coast, is a
strike of N. 19 degrees W. and S. 19 degrees E.: considering that in each
specified place my examination extended over an area of several miles, and
that Lieutenant Stokes' observations apply to a length of 100 miles, I
think this remarkable uniformity is pretty well established. The
prevalence, throughout the northern half of this line of coast, of a dip in
one direction, that is to the west, instead of being sometimes west and
sometimes east, is, judging from what I have elsewhere seen, an unusual
circumstance. In Brazil, La Plata, the Falkland Islands, and Tierra del
Fuego, there is generally an obvious relation between the axis of
elevation, the outline of the coast, and the strike of the cleavage or
foliation: in the Chonos Archipelago, however, neither the minor details of
the coast-line, nor the chain of the Cordillera, nor the subordinate
transverse mountain-axes, accord with the strike of the foliation and
cleavage: the seaward face of the numerous islands composing this
Archipelago, and apparently the line of the Cordillera, range N. 11 degrees
E., whereas, as we have just seen, the average strike of the foliation is
N. 19 degrees W.
There is one interesting exception to the uniformity in the strike of the
foliation. At the northern point of Tres Montes (latitude 45 degrees 52
minutes) a bold chain of granite, between two and three thousand feet in
height, runs from the coast far into the interior, in an E.S.E. line, or
more strictly E. 28 degrees S. and W. 28 degrees N. (In the distance, other
mountains could be seen apparently ranging N.N.E. and S.S.W. at right
angles to this one. I may add, that not far from Vallenar Bay there is a
fine range, apparently of granite, which has burst through the mica-slate
in a N.E. by E. and S.W. by S. line.) In a bay, at the northern foot of
this range, there are a few islets of mica-slate, with the folia in some
parts horizontal, but mostly inclined at an average angle of 20 degrees to
the north. On the northern steep flank of the range, there are a few
patches (some quite isolated, and not larger than half a-crown!) of the
mica-schist, foliated with the same northerly dip. On the broad summit, as
far as the southern crest, there is much mica-slate, in some places even
400 feet in thickness, with the folia all dipping north, at angles varying
from 5 degrees to 20 degrees, but sometimes mounting up to 30 degrees. The
southern flank consists of bare granite. The mica-slate is penetrated by
small veins of granite, branching from the main body. (The granite within
these veins, as well as generally at the junction with the mica-slate, is
more quartzose than elsewhere. The granite, I may add, is traversed by
dikes running for a very great length in the line of the mountains; they
are composed of a somewhat laminated eurite, containing crystals of
feldspar, hornblende, and octagons of quartz.) Leaving out of view the
prevalent strike of the folia in other parts of this Archipelago, it might
have been expected that they would have dipped N. 28 degrees E., that is
directly from the ridge, and, considering its abruptness, at a high
inclination; but the real dip, as we have just seen, both at the foot and
on the northern flank, and over the entire summit, is at a small angle, and
directed nearly due north. From these considerations it occurred to me,
that perhaps we here had the novel and curious case of already inclined
laminae obliquely tilted at a subsequent period by the granitic axis. Mr.
Hopkins, so well known from his mathematical investigations, has most
kindly calculated the problem: the proposition sent was,--Take a district
composed of laminae, dipping at an angle of 40 degrees to W. 19 degrees S.,
and let an axis of elevation traverse it in an E. 28 degrees S. line, what
will the position of the laminae be on the northern flank after a tilt, we
will first suppose, of 45 degrees? Mr. Hopkins informs me, that the angle
of the d
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