ON ITS EXTEBIOR. VOLCANOES. 289 



one erupts only loose scorise ; the other, lava flowing in 

 narrow streams. Moreover in many cases these character- 

 istic processes do not appear to have been always the same 

 at different epochs. Rarity or absence of lava-streams 

 does not characterise either continent so as to dis- 

 tinguish it from the other. Particular lava-streams 

 may escape recognition from a variety of circumstances. 

 Amongst these may be noticed an overlaying covering 

 of tufa, ashes (rapilli), or pumice; the confluence, 

 either at the same or different epochs, of several 

 currents forming an extended lava-field, or space 

 covered with fragments: and in an extensive plain, 

 little cones of eruption, from which as at Lancerote 

 streams of lava may have flowed, may have been 

 destroyed. In the earliest conditions of our unequally 

 cooling planet, in the first foldings of its surface, it 

 appears to me very probable that there may have been 

 an abundant flowing out of semifluid trachytic and 

 doleritic masses, of pumice, and of perlite containing 

 obsidian, from a complicated network of fissures above 

 which no volcanic framework had yet arisen. The 

 problem of such outpourings from simple fissures de- 

 serves the attention of geologists. 



In the Mexican series of volcanoes the greatest and, 

 since my American journey, the most celebrated phe- 

 nomenon of upheaval is the elevation of the newly 

 appeared Jorullo, and the emission of lava from it. 

 This volcano, whose topography, established by measure- 

 ments, I was the first to make known( 427 ), presents by its 

 position, intermediate between the two volcanoes of 

 Toluca and Colima, and by its having broken forth over 

 the great fissure of volcanic activity ( 42S ) which extends 



VOL. iv. u 



