ON ITS EXTERIOR. VOLCANOES. 307 



a long rocky ridge, on the southern extremity of which 

 stands the cubical rock (la Pena) whose shape has 

 given occasion to the ancient Aztec name Nauhcampa- 

 tepetl. I found, on my ascent of the mountain, no 

 trace of a fallen in crater, or of lateral eruption-open- 

 ings; nor did I find any masses of scoriae, or any 

 obsidian, pearlstone, or pumice belonging to it. The 

 blackish-grey rock is very uniform, composed of much 

 hornblende and a kind of felspar which is not glassy 

 felspar (sanidine), but oligoclase, which would stamp 

 the whole of the rock which is not porous, as a dioritic 

 trachyte. I describe the impressions which I imbibed. 

 If the substances which cover this black desolate 

 Malpais, on which I have dwelt purposely as opposing 

 too partial views of volcanic manifestations of force 

 from the interior, have not flowed from a lateral open- 

 ing in the Cofre de Perote itself, yet the upheaval of 

 that isolated mountain, 13,550 feet high, may have 

 occasioned the origination of the Loma de Tablas. At 

 such an upheaval, longitudinal fissures and a ramifying 

 fissured network may have been opened for a con- 

 siderable distance around by the "folding" of the 

 surface ; and from these fissures molten substances may 

 have flowed directly, either as dense masses or as 

 scoriaceous lavas, without the intervention or formation 

 of the framework of open cones, or of elevation-craters. 

 We know that we may look in vain, in the great moun- 

 tains of basalt and porphyritic schist, for either central 

 points (crater mountains) or lower ridge-encircled 

 hollows, to which their common origin might be ascribed. 

 Science gains by the most careful distinction between 

 whatever is genetically different in phenomena, 



