That Which In Shelly Or Neptunean Rocks Is Caused By The Action Of The Waters, Appears Sometimes To Be In The Volcanic Rocks The Effect Of Gaseous Emanations* Acting In The Direction Where They Find The Least Resistance.

(* At Vesuvius, the Duke de la Torre showed me, in 1805, in currents of recent lava, cavities extending in the direction of the current, six or seven feet long and three feet high.

These little volcanic caverns were lined with specular iron, which cannot be called oligiste iron, since M. Gay-Lussac's last experiments on the oxides of iron.) When melted matter moves on a very gentle slope, the great axis of the cavity formed by the elastic fluids is nearly horizontal, or parallel to the plane on which the movement of transition takes place. A similar disengagement of vapours, joined to the elastic force of the gases, which penetrate strata softened and raised up, appears sometimes to have given great extent to the caverns found in trachytes or trappean porphyries. These porphyritic caverns, in the Cordilleras of Quito and Peru, bear the Indian name of Machays.* (* Machay is a word of the Quichua language, commonly called by the Spaniards the Incas' language. Callancamachay means a cavern as large as a house, a cavern that serves as a tambo or caravansarai.) They are in general of little depth. They are lined with sulphur, and differ by the enormous size of their openings from those observed in volcanic tufas* in Italy, at Teneriffe, and in the Andes. (*Sometimes fire acts like water in carrying off masses, and thus the cavities may be caused by an igneous, though more frequently by an aqueous erosion or solution.) It is by connecting in the mind the primitive, secondary, and volcanic rocks, and distinguishing between the oxidated crust of the globe, and the interior nucleus, composed perhaps of metallic and inflammable substances, that we may account for the existence of grottoes everywhere. They act in the economy of nature as vast reservoirs of water and of elastic fluids.

The gypseous caverns glitter with crystallized selenites. Vitreous crystallized plates of brown and yellow stand out on a striated ground composed of layers of alabaster and fetid limestone. The calcareous grottoes have a more uniform tint. They are more beautiful, and richer in stalactites, in proportion as they are narrower, and the circulation of air is less free. By being spacious, and accessible to air, the cavern of Caripe is almost destitute of those incrustations, the imitative forms of which are in other countries objects of popular curiosity. I also sought in vain for subterranean plants, those cryptogamia of the family of the Usneaceae, which we sometimes find fixed on the stalactites, like ivy on walls, when we penetrate for the first time into a lateral grotto.* (* Lichen tophicola was discovered when the beautiful cavern of Rosenmuller in Franconia was first opened. The cavity containing the lichen was found closed on all sides by enormous masses of stalactite.)

The caverns in mountains of gypsum often contain mephitic emanations and deleterious gases.

Enter page number Go  Previous Next
Page 224 of 407
Words from 115798 to 116309 of 211363