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These ruin-like rocks command the plain; they are composed of a coarse-grained granite, nearly porphyritic, the yellowish white feldspar crystals of which are more than an inch and a half long. Mica is rare in them, and is of a fine silvery lustre. Nothing can be more picturesque and solemn than the aspect of this group of mountains, half covered with vegetation.
Porphyritic tuffs and massive limestone compose the western chain of the Andes above Lima, while in the Oroya Valley we find carbonaceous sandstones. Some of the tuffs may be of the Jurassic age, though the Cretaceous period is also largely represented.
The gypseous formation, like that of the porphyritic breccia- conglomerate on which it rests, is of inconsiderable breadth; though of greater breadth in Northern than in Central Chile.
This regular disposition of lithoid basaltic lava and feldsparry vitreous lava is analogous to the phenomena of all trappean mountains; it reminds us of those phonolites lying in very ancient basalts, those close mixtures of augite and feldspar which cover the hills of wacke or porous amygdaloids: but why are the porphyritic or feldsparry lavas of the Peak found only on the summit of the volcano?
In the kingdom of Quito, the great catastrophe of Riobamba led several well-informed persons to think that that country would be less frequently disturbed, if the subterranean fire should break the porphyritic dome of Chimborazo; and if that colossal mountain should become a burning volcano. At all times analogous facts have led to the same hypotheses.
Hence the points of eruption must have been situated westward of the present range, in the direction of the main Cordillera: as, however, the flanks of the Cordillera are entirely composed of various porphyries, chiefly claystone and greenstone, some intrusive, and others belonging to the porphyritic conglomerate formation, but all quite unlike these submarine lava-streams, we must in all probability look to the plain of Uspallata for the now deeply buried points of eruption.
I am tempted briefly to describe three other singular allied varieties of rock; the first without examination would have passed for a stratified porphyritic breccia, but all the included angular fragments consisted of a border of pinkish crystalline feldspathic matter, surrounding a dark translucent siliceous centre, in which grains of quartz not quite blended into the paste could be distinguished: this uniformity in the nature of the fragments shows that they are not of mechanical, but of concretionary origin, having resulted perhaps from the self-breaking up and aggregation of layers of indurated tuff containing numerous grains of quartz, into which, indeed, the whole mass in one part passed.
Next we find, in the same county, that elvans, or straight dikes of porphyritic granite, have cut through the granite and the veins before mentioned, but have not penetrated the Devonian rocks.
Before going to see the pyramids, we visited the caves in the hill-side not far from them, whence the stone was brought to build them. It is tetzontli, the porous amygdaloid which abounds among the porphyritic hills, a beautiful building-stone, easily worked, and durable. There was a large space that seemed to have been quarried out bodily, and into this opened numerous caves.
After leaving the plutonic hills near the town, I met first, as in the main valley, with the gypseous formation, having the same diversified character as before, and soon afterwards with masses of porphyritic conglomerate, about one thousand feet in thickness.
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