TRUE VOLCANOES. 348 



Cotonaxi : 32 miles from the former, and about half that dis- 

 tance from the latter. They are reached by a gallery. The 

 workmen assert that from the horizontal solid layers, of 

 which a few are surrounded by loamy pumice fragments, 

 quadrangular blocks of 20 feet, divided by no transverse fis- 

 sures, might be procured. The pumice-stone, which is partly 

 white and partly bluish gray, consists of very fine and long 

 fibres, with a silky lustre. The parallel fibres have some- 

 times a knotted appearance, and then exhibit a singular 

 structure. The knots are formed by roundish particles of 

 finely porous pumice-stone, from 1 1^ line in breadth, 

 around which long fibres curve so as to inclose them. 

 Brownish black mica in small six-sided tables, white 

 crystals of oligoclase, and black hornblende are sparingly 

 scattered in it ; on the other hand, the glassy felspar, which 

 elsewhere (Camaldoli, near Naples) occurs in pumice-stone, 

 is entirely wanting. The pumice-stone of Cotopaxi is very 

 different from that of the quarries of Zumbalica 33 : its fibres 

 are short, not parallel, but curved in a confused man- 

 ner. Magnesia-mica, however, is not peculiar to pumice- 

 stone, for it is also found in the fundamental mass of the 

 trachyte 34 of Cotopaxi. At the more southern volcano, 

 Tungurahua, pumice-stone appears to be entirely wanting. 

 There is no trace of obsidian in the vicinity of the quar- 



33 This difference has also been recognized by the acute Abich, 

 (Ueber Natur und Zusammenhang vulkanischer B'ddunyen, 1841, 

 s. 83). 



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

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

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

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

 mountain in masses in the opposite Cordilleras. In the specimens col- 

 lected by me in 1802, and by Boussingault in 1831, the fundamental 

 mass is partly light or greenish gray, with a pitchstone-like lustre and 

 translucent at the edges ; partly black, nearly resembling basalt, with 

 large and small pores, which possess shining walls. The inclosed oligo- 

 clase is distinctly limited ; sometimes in very brilliant crystals, very dis- 

 tinctly 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 con- 

 taining much oligoclase, there was some native sulphur, probably 

 deposited by the all- penetrating sulphurous vapours. 



