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Updated: June 23, 2025
Piquillacta is a large mass, nearly five miles long and two miles wide, rising to a point of 2000 feet above the Huatanay and Vilcanota rivers, it has no streams, brooks, or springs. It is an isolated, extinct volcano surrounded by igneous rocks, lavas, andesites, and basalts.
From the points of structure in the central ridge, now enumerated, namely, from the convergence towards it of the beds of the upper series, from the lavas there becoming highly cellular, from the flat ledge, extending along its inner and precipitous side, like that within some still active craters, from the parapet-like wall on its summit, and lastly, from its peculiar curvature, unlike that of any common line of elevation, I cannot doubt that this curved ridge forms the last remnant of a great crater.
Of the districts to be described, the first which claims our notice is the Deccan. The vast area thus circumscribed is far from representing the original extent of the tract overspread by the lava-floods, as outlying fragments of these lavas are found as far east as long. 84° E. in one direction, and at Kattiwar and Cutch in another.
When cut in one direction it is of a beautiful jetty black; when cut across that direction it is glistering gray. The lavas of Vesuvius are generally of a brown colour, and are also used in the arts. In them are found the beautiful olive-green crystals of the mineral called olivine, sometimes used by jewellers.
What is rather remarkable, these five, together with Jorullo, all lie nearly in a straight line running east and west. The tracts of country which these volcanoes have desolated with their lavas are called by the Mexicans the "Malpays." The most remarkable of these mountains is Popocatepetl.
With regard to Etna, I have shown, from observations made by me in 1857, that modern lavas, several of them of known date, have formed continuous beds of compact stone even on slopes of 15, 36, and 38 degrees, and, in the case of the lava of 1852, more than 40 degrees. The thickness of these tabular layers varies from 1 1/2 foot to 26 feet.
So ancient are the latest floods in the Columbia basin that they have weathered to a residual yellow clay from thirty to sixty feet in depth and marvelously rich in the mineral substances on which plants feed. In the Snake River valley the latest lavas are much younger.
A painter had laid heaven open; he beheld the Virgin Mary wrapped in a golden cloud among the angels, shining more brightly than the sun, receiving the prayers of sufferers, on whom this second Eve Regenerate smiles pityingly. At the touch of a mosaic, made of various lavas from Vesuvius and Etna, his fancy fled to the hot tawny south of Italy.
Over the marine lavas and tuffs trachytic and basaltic products of subaerial volcanic origin, between 4000 and 5000 feet in thickness, have been piled, the central parts of the Grand Canary reaching the height of about 6000 feet above the level of the sea.
In explanation of this phenomenon, M. Necker refers us to Sir W. Hamilton's account of an eruption of Vesuvius in the year 1779, who records the following fact: "The lavas, when they either boiled over the crater, or broke out from the conical parts of the volcano, constantly formed channels as regular as if they had been cut by art down the steep part of the mountain; and whilst in a state of perfect fusion, continued their course in those channels, which were sometimes full to the brim, and at other times more or less so, according to the quantity of matter in motion.
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