444 REACTION OF THE INTERIOR OF THE EARTH 



me from the Cerro de las Navajas, on the eastern 

 declivity of Jacal ; those of Chico with many crystals of 

 mica ; those of Zimapan, S. S. W. of the city of Mexico, 

 with distinct, interspersed, small quartz crystals; the 

 pumices of the Rio Mayo (on the mountain-path from 

 Popayan to Pasto), and of the extinct volcano of Sorata 

 near Popayan. The subterranean quarries of pumice- 

 stone near Llactagunga ( 635 ) contain much mica, oligo- 

 clase, and (which is very rare in pumice-stones and 

 obsidian) also hornblende ; the latter has, however, also 

 been seen in the pumice of the volcano of Arequipa. 

 Common felspar, orthoclase, is never found in pumice 

 together with sanidine ; augites are also wanting. The 

 Somma, but not the cone of Vesuvius itself, contains 

 pumice which encloses earthy masses of carbonate of 

 lime. It is by the same remarkable variety of a calca- 

 reous pumice that Pompeii has been overwhelmed. ( 636 ) 

 Obsidians are rare in true lava-currents, having been 

 found almost exclusively in those of the Peak of Teneriffe, 

 Lipari, and Volcano. 



Passing to the association of obsidian and pumice in 

 one and the same volcano, we find the following facts : 

 Pichincha has large fields of pumice, and no obsidian. 

 Chimborazo, like Etna whose trachytes, however, have 

 quite a different composition (they contain labradorite 

 instead of oligoclase), shows neither obsidian nor 

 pumice. I also remarked their absence in ascending 

 Tungurahua. The volcano of Purace, near Popayan, has 

 much obsidian intermixed in its trachytes, and has never 

 produced pumice. Large plains, as those from which Ili- 

 nissa, Carguairazo, and the "Altar " rise, are covered with 

 pumice. The subterranean pumice quarries near Llacta- 



