TRUE VOLCANOES. 291 



as the lowly Stromboli ; of two neighboring volcanoes, one 

 throws out pumice-stone without obsidian, the other both at 

 once ; one furnishes only loose cinders, the other lava flow- 

 ing in narrow streams. These characteristic processes, more- 

 over, in many volcanoes appear not to have been always the 

 same at various epochs of their activity. To neither of the 

 two continents is rarity or total absence of lava streams to 

 be peculiarly ascribed. Remarkable distinctions only occur 

 in those groups with regard to which we must confine our- 

 selves to definite historical periods near to our own times. 

 The non-detection of single lava streams depends simultane- 

 ously upon many conditions. Among these we may instance 

 the deposition of Vast layers of tufa, rapilli, and pumice-stone ; 

 the simultaneous and non-simultaneous confluence of several 

 streams, forming a widely-extended lava-field covered with 

 detritus; the circumstance that in a wide plain the small 

 conical eruptive cones, the volcanic platform, as it were, from 

 which, as at Lancerote, the lava had flowed forth in streams, 

 have long since been destroyed. In the most ancient condi- 

 tions of our unequally-cooling planet, in the earliest foldings 

 of its surface, it appears to me very probable that a frequent 

 viscid outflow of trachy tic and doleritic rocks, of masses of 

 pumice-stone or perlite, containing obsidian, took place from 

 a composite net-work of fissures, over which no platform has 

 ever been elevated or built up. The problem of such simple 

 effusions from fissures deserves the attention of geologists. 



In the series of Mexican volcanoes, the greatest and, since 

 ray American travels, the most celebrated phenomenon, is the 

 elevation of the newly-produced Jorullo, and its effusion of 

 lava. This volcano, the topography of which, founded on 

 measurements, I was the first to make known,* by its posi- 

 tion between the two volcanoes of Toluca and Colima, and 

 by its eruption on the great fissure of volcanic activity, f 

 which extends from the Atlantic Ocean to the Pacific, pre- 

 sents an important geognostic phenomenon, which has con- 

 sequently been all the more the subject of dispute. Follow- 

 ing the vast lava stream which the new volcano poured out, 

 I succeeded in getting far into the interior of the crater, and 

 in establishing instruments there. The eruption in a broad 

 and long-peaceful plain in the former province of Michuacan, 

 in the night from the 28th to the 29th of September, 1759, 

 at a distance of more than 120 miles from any other volcano, 



* Atlas Gioyraphique et Physique, accompanying the Relation Hie- 

 torique, 1814, pi. 28 and 29. * f Cosmos, vol. v., p. 264-266. 



