olour
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 conc