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The major part of the wall is well built of rough stones, laid in clay, while the sides of the gateway are faced with carefully cut andesite ashlars of an entirely different style.
The andesite is covered by strata of dark-coloured, crystalline, obscurely porphyritic rocks, and above them by the ordinary porphyritic conglomerates, the strata all dipping away at a small angle from the underlying mass. The surrounding lofty mountains appear to be entirely composed of the porphyritic conglomerate, and I estimated its thickness here at between six and seven thousand feet.
As shown by the map in the Report of the Royal Society, the group of islands which existed previous to 1883 were but the unsubmerged portions of one vast volcanic crater, built up of a remarkable variety of lava allied to the andesite of the Java volcanoes, but having a larger percentage of silica, and hence falling under the head of "enstatite-dacite."
The earliest volcanic eruptions occurred in the later Eocene epoch and were continued into the succeeding Miocene stage. These consisted of rocks moderately rich in silica, and are grouped under the heads of propylite and andesite. To these succeeded during the Pliocene epoch still more highly silicated rocks of trachytic type, consisting of sanidine and oligoclase trachytes.
Near Illapel, after passing for some distance over granite, andesite, and andesitic porphyry, we come to a greenish stratified feldspathic rock, which I believe is altered clay-slate, conformably capped by porphyries and porphyritic conglomerate of great thickness, dipping at an average angle of 20 degrees to N.E. by N. The uppermost beds consist of conglomerates and sandstone only a little metamorphosed, and conformably covered by a gypseous formation of very great thickness, but much denuded.
"The andesite volcanoes were mainly located along the crest of the Sierra, in fact, almost continuously from Thompson Peak, west of Honey Lake, down to latitude 38° degrees 10'. Farther south the eruptions diminished greatly in intensity.... Along the first summit of the range west of Tahoe the greatest number of vents are found.
Central Chile. Basal formations of the Cordillera. Origin of the porphyritic clay-stone conglomerate. Andesite. Volcanic rocks. Section of the Cordillera by the Peuquenes are Portillo Pass. Great gypseous formation. Peuquenes line; thickness of strata, fossils of. Portillo line. Conglomerate, orthitic granite, mica-schist, volcanic rocks of.
From the sea to the town of Copiapo, a distance estimated at thirty miles, the mountains are composed of greenstone, granite, andesite, and blackish porphyry, together with some dusky-green feldspathic rocks, which I believe to be altered clay-slate: these mountains are crossed by many brown-coloured dikes, running north and south.
I did not meet in this valley with any true andesite, but only with imperfect andesitic porphyry, including large crystals of hornblende: numerous as have been the varieties of intrusive porphyries already mentioned, there were here mountains composed of a new kind, having a compact, smooth, cream-coloured basis, including only a few crystals of feldspar, and mottled with dendritic spots of oxide of iron.
The derivation of the water and the exact mode of eruption are difficult to determine.... Towards the summits the breccias gradually lose their stratified character and become more firmly cemented. Over large areas in the Truckee quadrangle the andesite masses consist of breccias containing numerous dykes and necks of massive andesite....
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