ON ITS EXTERIOK. VOLCANOES. 321 



and only 16 from the latter. They are reached by a 

 gallery. The workmen assured me, that square blocks 

 of 20 or 21 feet could be obtained entire and without 

 cross cracks, from the horizontal solid beds, of which a 

 few are surrounded by muddy pumice detritus. The 

 pumice-stone, partly white and partly bluish grey, is of 

 very fine texture, and has long fibres of a silky lustre. 

 The parallel fibres have sometimes a knotted appearance, 

 and they then show a curious structure. The knots 

 are formed by roundish morsels of finely porous pumice, 

 about a tenth of an inch thick, round which the long 

 fibres curve so as to enclose them. Brownish black 

 mica, in six-sided small tablets, white oligoclase crystals, 

 and black hornblende, are scantily interspersed ; but no 

 glassy felspar, which elsewhere, as at Camaldoli, near 

 Naples, is found in pumice-stone. The pumice-stone of 

 Cotopaxi differs from that of the Zumbalica quarries 

 ( 457 ) : its fibres are short, not parallel, but confusedly 

 bent. Magnesia-mica is not, however, peculiar to the 

 pumice-stones, it is also found in the trachyte of 

 Cotopaxi. ( 458 ) The volcano of Tungurahua appears to 

 be entirely wanting in pumice. There is no trace of 

 obsidian in the quarries of Zumbalica, but in the blocks 

 thrown out by Cotopaxi, and lying near Mulalo, I have 

 found, in very large masses, black obsidian, having a 

 conchoidal fracture embedded in bluish grey weathered 

 pearlstone. Specimens are preserved in the Royal 

 Mineralogical Collection at Berlin. It would appear, 

 therefore, that the quarries of pumice-stone 16 miles from 

 the foot of Cotopaxi, which have been described above, 

 are not at all related to that mountain in mineralogical 

 constitution, and only stand towards it in the same 

 VOL. iv. Y 



