Geological Observations on South AmericaDarwin, Charles
Science
Geological Observations on South America
Darwin, Charles
Beagle Expedition (1831-1836); Geology -- South America; Mollusks, Fossil -- South America; Paleontology
Seventhly: strata, eighty feet in thickness, of various indurated
tuffs, as before; many of the varieties have a fine basis including
rather coarse extraneous particles; some of them are compact and
semi-porcellanic, and include vegetable impressions.
Eighthly: a bed, about fifty feet thick, of greenish-grey, compact,
feldspathic lava, with numerous small crystals of opaque feldspar,
black augite, and oxide of iron. The junction with the bed on which it
rested, was ill defined; balls and masses of the feldspathic rock being
enclosed in much altered tuff.
Ninthly: indurated tuffs, as before.
Tenthly: a conformable layer, less than two feet in thickness, of
pitchstone, generally brecciated, and traversed by veins of agate and
of carbonate of lime: parts are composed of apparently concretionary
fragments of a more perfect variety, arranged in horizontal lines in a
less perfectly characterised variety. I have much difficulty in
believing that this thin layer of pitchstone flowed as lava.
Eleventhly: sedimentary and tufaceous beds as before, passing into
sandstone, including some conglomerate: the pebbles in the latter are
of claystone porphyry, well rounded, and some as large as
cricket-balls.
Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of
bed No. 8, divided by numerous joints into large angular blocks.
Thirteenthly: sedimentary beds as before.
Fourteenthly: a thick bed of greenish or greyish black, compact basalt
(fusing into a black enamel), with small crystals, occasionally
distinguishable, of feldspar and augite: the junction with the
underlying sedimentary bed, differently from that in most of the
foregoing streams, here was quite distinct:—the lava and tufaceous
matter preserving their perfect characters within two inches of each
other. This rock closely resembles certain parts of that varied and
singular lava-stream No. 6; it likewise resembles, as we shall
immediately see, many of the great upper beds on the western flank and
on the summit of this range.
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