TRUE VOLCANOES. 323 



lei, but curved in a confused manner. Magnesia mica, how- 

 ever, is not peculiar to pumice-stone, for it is also found in the 

 fundamental mass of the trachyte* of Cotopaxi. At the more 

 southern volcano, Tungurahua, pumice-stone appears to be 

 entirely wanting. There is no trace of obsidian in the vi- 

 cinity of the quarries of Zumbalica, but I have found black 

 obsidian with a conchoidal fracture in very large masses, im- 

 mersed in bluish-gray weathered perlite, among the blocks 

 thrown out from Cotopaxi, and lying near Mulalo. Of this 

 fragments are preserved in the Royal Collection of Minerals 

 at Berlin. The pumice-stone quarries here described, at a 

 distance of sixteen miles from the foot of Cotopaxi, appear, 

 therefore, to judge from their mineralogical nature, to be quite 

 foreign to that mountain, and only to stand in the same rela- 

 tion to it which all the volcanoes of Pasto and Quito, occu- 

 pying many thousand square miles, present to the volcanic 

 focus of the equatorial Cordilleras. Have these pumice-stones 

 been the centre and interior of a proper crater of elevation, 

 the external wall of which has been destroyed in the numer- 

 ous convulsions which the surface of the earth has here un- 

 dergone? or have they been deposited here upon fissures in 

 apparent rest during the most ancient foldings of the earth's 

 crust ? For the assumption of aqueous sedimentary alluvia, 

 such as are often exhibited in volcanic tufaceous masses mix- 

 ed with remains of plants and shells, is attended with still 

 greater difficulties. 



The same questions are suggested by the great mass of 

 pumice-stone, at a distance from all intumescent volcanic 

 platforms, which I found on the Rio Mayo, in the Cordillera 



* The rock of Cotopaxi has essentially the same mineralogical com- 

 position as that of the nearest volcanoes, Antisana and Tungurahua. 

 It is a trachyte, composed of oligoclase and augite, and consequently 

 a Chimborazo rock : a proof of the identity of the same kind of vol- 

 canic mountain in masses in the opposite Cordilleras. In the speci- 

 mens collected by me in 1802, and by Boussingault in 1831, the funda- 

 mental mass is partly light or greenish gray, with a pitch-stone-like 

 lustre and translucent at the edges; partly black, nearly resembling 

 basalt; with large and small pores, which possess shining walls. The 

 inclosed oligoclase is distinctly limited ; sometimes in very brilliant 

 crystals, very distinctly striated on the cleavage planes ; sometimes in 

 small fragments, and difficult of detection. The intermixed augites 

 are brownish and blackish green, and of very variable size. Dark 

 laminae of mica and black metallic grains of magnetic iron are rarely 

 and probably quite accidentally sprinkled through the mass. In the 

 pores of a mass containing much oligoclase there was some native sub 

 phur, probably deposited by the all-penetrating sulphurous vapors. 



