Page:Popular Science Monthly Volume 33.djvu/660

642 contained in the sedimentary beds originated chiefly from the interior reservoirs of the globe, whence it has also been brought by the agency of thermal springs. This is illustrated in the important beds of Estremadura, where phosphorite associated with quartz constitutes numerous vertical veins, which have been filled up from below. Incidentally, the substance has penetrated the calcareous strata, and has assumed the forms of fossils within them, thus bringing a new proof of aqueous precipitation.

Still more frequently than any other substance, is quartz distributed in large veins. The granitic plateau of France, Brittany, the Vosges, and the Pyrenees presents numerous examples of it. These veins can often be perceived from a considerable distance. Besides crystallized quartz, they frequently contain parcels of metalliferous minerals, and thus represent transitions toward metalliferous veins proper. The ribboned texture of chalcedony and agate, which abound in these veins, and the manner in which they are related to deposits of precisely the same nature, inclosed in adjoining strata, confirm the view of their aqueous origin and permit their age to be determined. This is exemplified in the department of the Loire, where these veins are thus associated with porphyry, and at the northern point of Morvan; and in the remarkable quartz-veins of the Sierra Nevada, in California, which are auriferous at some points. Comprised within a zone some nine or ten miles wide, they extend from north to south along the chain for nearly two hundred miles. One of the most considerable of these, "the great quartz-vein," can be followed over more than thirty-five miles.

In fact, all these outflowings of quartz and connected minerals, whatever may be the diversity of their forms, in veins, masses, or strata, attest, not less authentically than the metalliferous beds, the intervention and generative power of subterranean waters which have been long since exhausted. We shall next see that waters sufficiently superheated deposit as quartz-crystal the silica which they hold in solution. It is thus explained how this mineral has in a certain way become the binder-up of the fractures of the terrestrial crust.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.

A connection may be hypothetically traced between the frequent earthquakes in South America and certain subsidences which appear to be going on in the Andes. The city of Quito has sunk 26 feet in 122 years; the farm of Antisana—the highest inhabited spot on the globe—165 feet in 64 years; and the peak of Pichincha 218 feet in 122 years. The squeezing of the crust of the earth which is produced by such sinking of the masses of these ranges must produce violent dislocations in the surrounding regions; and these are the terrible earthquakes which we witness.