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Updated: June 18, 2025
The central great mass of this latter rock, is covered on the eastern side by a black, fine-grained, highly micaceous slate, which, together with the succeeding mountains of porphyry, are traversed by numerous white dikes, branching from the andesite, and some of them extending in straight lines, to a distance of at least two miles.
These hillocks consist of purple claystone and of various other porphyries, and of much white feldspathic greenstone passing into andesite; this latter variety included in one case crystals of orthitic and albitic feldspar touching each other, and others of hornblende, chlorite, and epidote.
The volcano of Charchani, lying to the north of Arequipa, reaches an elevation of 18,382 Parisian feet. That of Pichupichu reaches a height of 17,355 Par. feet. The central cone of Misti has been variously estimated to range from 17,240 to 19,000 Par. feet. The rocks of which the mountains are composed consist of varieties of andesite.
Being in hard andesite rock from top to bottom, the ground does not swell and crowd upon the timbers. If it shall be thought advisable to go to a greater depth than 3,200 feet, a station of large size will be made on the east side of the present shaft, and in this station will be sunk a shaft of smaller size.
Proceeding up the valley we come to another north and south line of granite, andesite, and blackish porphyry, which seem to lie in an irregular trough of the porphyritic conglomerate. Again, on the south side of the R. Claro, there are some irregular granitic hills, which have thrown off the strata of porphyritic conglomerate to the N.W. by W.; but the stratification here has been much disturbed.
I saw also some syenite and one mass which resembled andesite, but of which I likewise neglected to collect specimens. Caldcleugh, however, collected here specimens of ribboned jasper, magnesian limestone, and other minerals.
These dikes are composed of green, white, and blackish rocks, all porphyritic with feldspar, and often with large crystals of hornblende. The white varieties approach closely in character to andesite, which composes as we have seen, the injected axes of so many of the lines of elevation. Some of the green varieties are finely laminated, parallel to the walls of the dikes.
Rose, Marlette Peak, Glenbrook and the pass behind it, are all in sight and the Lake glistening in pearly brilliancy below. At the end of the Squaw Peak ridge, on the right, is a mass of andesite, looking like rude cordwood, and just above is a mass of breccia very similar to that found in the Truckee Valley a few miles below Tahoe Tavern.
On the summits nearer to Tahoe the volcanic outflows were of andesite, a rough and porous rock of dark gray to dark brown color. Lindgren says: "By far the greater part of the andesite occurs in the form of a tuffaceous breccia in numerous superimposed flows.
Rocca Monfina, in Southern Italy, showing a crater-ring of trachytic tuffs, from the midst of which, according to Judd, an andesite lava-cone has been built up.
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