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