Note 60 — Chapter VII

"As a general rule, whenever the mass of mountains rises much above the limit of perpetual snow, the primitive rocks disappear, and the summits are trachyte or trappean porphyry."—Humboldt. In general, "the great Cordilleras are formed of innumerable varieties of granites, gneiss, schists, hornblende, chloritic slates, porphyries, etc., and these rocks alternate with each other in meridional bands, which in the ridges frequently present the appearance of a radiated or fan-shaped structure, and under the plains are more or less vertical."—Evan Hopkins, F.G.S.