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Updated: July 17, 2025
This band of symmetrically troubled country is about eight miles in width. The island of Quiriquina, in the Bay of Concepcion, is formed of various soft and often ferruginous sandstones, with bands of pebbles, and with the lower strata sometimes passing into a conglomerate resting on the underlying metamorphic schists.
After following the stream for two days we were obliged to retrace our steps to Seal Lake, thereafter keeping to the course pursued on the inland journey. The numbers following the names of rocks refer to corresponding numbers in appendix. Of the rocks observed, by far the greater number are foliated basic eruptives, schists and gneisses.
Down by the river the walls are composed of black gneiss, slates, and schists, all greatly implicated and traversed by dikes of granite. Let this formation be called the black gneiss. It is usually about 800 feet in thickness. Then over the black gneiss are found 800 feet of quartzites, usually in very thin beds of many colors, but exceedingly hard, and ringing under the hammer like phonolite.
In the Black Hills the irruption of an immense mass of granite has caused or accompanied the upheaval of pre-Cambrian strata and metamorphosed them by heat and pressure into gneisses, schists, quartzites, and slates.
Besides these, however, the same ancient schists have yielded two genera of brachiopods, Orthis and Orbicula, a Pteropod of the genus Theca, and four echinoderms of the cystidean family.
Yet I asked myself if I was not depending too much on this change in the rock. Might I not myself be mistaken? Were we really crossing the layers of rock which overlie the granite foundation? The name given by Sir Roderick Murchison to a vast series of fossiliferous strata, which lies between the non-fossiliferous slaty schists below and the old red sandstone above.
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.
The "Aeasy," a stream of 15 yards broad and thigh deep, came down alongside our precipitous path, and formed cascades by leaping 300 feet at a time. These, with the bright red of the clay schists among the greenwood-trees, made the dullest of my attendants pause and remark with wonder.
Plutonic action would be very general, and volcanic mud, ash, and sand would be ejected and spread far and wide, which, sinking to the bottom of the water, may possibly be the origin of what we now designate the azoic or metamorphic slates and schists, as also the early Cambrian and Silurian strata.
Two leagues beyond Piri, at Salapunco, the road skirts the base of precipitous cliffs, the beginnings of a wonderful mass of granite mountains which have made Uilcapampa more difficult of access than the surrounding highlands which are composed of schists, conglomerates, and limestone.
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