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I have just returned from Mendoza, having crossed the Cordilleras by two
passes. This trip has added much to my knowledge of the geology of the
country. Some of the facts, of the truth of which I in my own mind feel
fully convinced, will appear to you quite absurd and incredible. I will
give a very short sketch of the structure of these huge mountains. In
the Portillo pass (the more southern one) travellers have described
the Cordilleras to consist of a double chain of nearly equal altitude
separated by a considerable interval. This is the case; and the same
structure extends to the northward to Uspallata; the little elevation
of the eastern line (here not more than 6,000-7,000 feet.) has caused it
almost to be overlooked. To begin with the western and principal
chain, we have, where the sections are best seen, an enormous mass of a
porphyritic conglomerate resting on granite. This latter rock seems
to form the nucleus of the whole mass, and is seen in the deep
lateral valleys, injected amongst, upheaving, overturning in the most
extraordinary manner, the overlying strata. The stratification in all
the mountains is beautifully distinct and from a variety in the colour
can be seen at great distances. I cannot imagine any part of the world
presenting a more extraordinary scene of the breaking up of the crust
of the globe than the very central parts of the Andes. The upheaval has
taken place by a great number of (nearly) N. and S. lines; which in most
cases have formed as many anticlinal and synclinal ravines; the strata
in the highest pinnacles are almost universally inclined at an angle
from 70 deg to 80 deg. I cannot tell you how I enjoyed some of these
views--it is worth coming from England, once to feel such intense
delight; at an elevation from 10 to 12,000 feet there is a transparency
in the air, and a confusion of distances and a sort of stillness which
gives the sensation of being in another world, and when to this is
joined the picture so plainly drawn of the great epochs of violence, it
causes in the mind a most strange assemblage of ideas.
The formation I call Porphyritic Conglomerates is the most important and
most developed one in Chili: from a great number of sections I find it
a true coarse conglomerate or breccia, which by every step in a slow
gradation passes into a fine claystone-porphyry; the pebbles and cement
becoming porphyritic till at last all is blended in one compact rock.
The porphyries are excessively abundant in this chain. I feel sure at
least 4/5ths of them have been thus produced from sedimentary beds in
situ. There are porphyries which have been injected from below amongst
strata, and others ejected, which have flowed in streams; it is
remarkable, and I could show specimens of this rock produced in these
three methods, which cannot be distinguished. It is a great mistake
considering the Cordilleras here as composed of rocks which have flowed
in streams. In this range I nowhere saw a fragment, which I believe to
have thus originated, although the road passes at no great distance
from the active volcanoes. The porphyries, conglomerate, sandstone and
quartzose sandstone and limestones alternate and pass into each
other many times, overlying (where not broken through by the granite)
clay-slate. In the upper parts, the sandstone begins to alternate with
gypsum, till at last we have this substance of a stupendous thickness.
I really think the formation is in some places (it varies much) nearly
2,000 feet thick, it occurs often with a green (epidote?) siliceous
sandstone and snow-white marble; it resembles that found in the Alps in
containing large concretions of a crystalline marble of a blackish grey
colour. The upper beds which form some of the higher pinnacles consist
of layers of snow-white gypsum and red compact sandstone, from the
thickness of paper to a few feet, alternating in an endless round.
The rock has a most curiously painted appearance. At the pass of the
Peuquenes in this formation, where however a black rock like clay-slate,
without many laminae, occurring with a pale limestone, has replaced the
red sandstone, I found abundant impressions of shells. The elevation
must be between 12 and 13,000 feet. A shell which I believe is the
Gryphaea is the most abundant--an Ostrea, Turratella, Ammonites, small
bivalves, Terebratulae (?). Perhaps some good conchologist (6/1. Some of
these genera are mentioned by Darwin ("Geol. Obs." page 181) as having
been named for him by M. D'Orbigny.) will be able to give a guess, to
what grand division of the formations of Europe these organic remains
bear most resemblance. They are exceedingly imperfect and few. It
was late in the season and the situation particularly dangerous for
snow-storms. I did not dare to delay, otherwise a grand harvest might
have been reaped. So much for the western line; in the Portillo pass,
proceeding eastward, we meet an immense mass of conglomerate, dipping to
the west 45 deg, which rest on micaceous sandstone, etc., etc., upheaved
and converted into quartz-rock penetrated by dykes from the very
grand mass of protogine (large crystals of quartz, red feldspar, and
occasional little chlorite). Now this conglomerate which reposes on and
dips from the protogene 45 deg consists of the peculiar rocks of the
first described chain, pebbles of the black rock with shells, green
sandstone, etc., etc. It is hence manifest that the upheaval (and
deposition at least of part) of the grand eastern chain is entirely
posterior to the western. To the north in the Uspallata pass, we have
also a fact of the same class. Bear this in mind: it will help to
make you believe what follows. I have said the Uspallata range is
geologically, although only 6,000-7,000 feet, a continuation of the
grand eastern chain. It has its nucleus of granite, consists of grand
beds of various crystalline rocks, which I can feel no doubt are
subaqueous lavas alternating with sandstone, conglomerates and white
aluminous beds (like decomposed feldspar) with many other curious
varieties of sedimentary deposits. These lavas and sandstones alterate
very many times, and are quite conformable one to the other. During two
days of careful examination I said to myself at least fifty times, how
exactly like (only rather harder) these beds are to those of the
upper Tertiary strata of Patagonia, Chiloe and Concepcion, without
the possible identity ever having occurred to me. At last there was
no resisting the conclusion. I could not expect shells, for they never
occur in this formation; but lignite or carbonaceous shale ought to
be found. I had previously been exceedingly puzzled by meeting in
the sandstone, thin layers (few inches to feet thick) of a brecciated
pitchstone. I strongly suspect the underlying granite has altered
such beds into this pitchstone. The silicified wood (particularly
characteristic) was yet absent. The conviction that I was on the
Tertiary strata was so strong by this time in my mind, that on t |