308 COSMOS. 



internal constitution of volcanoes, and, which I must add, has 

 not been treated with sufficient earnestness, has recently been 

 so often spoken of. the present appears a fitting place in which 

 to bring it under a more general point of view. Although 

 it is 1 very probable that, in a group or series of volcanoes all 

 the members stand in a certain common relation to the 

 general focus, the molten interior of the earth, still each 

 individual presents peculiar physical and chemical processes 

 as regards strength and frequency of activity, degree and 

 form of fluidity, and material difference of products, pecu- 

 liarities which cannot be explained by the comparison of the 

 form, and elevation above the present surface of the sea. 

 The gigantic mountain, Sangay, is as uninterruptedly 

 active as the lowly Stromboli ; of two neighbouring vol- 

 canoes, one throws out pumice-stone without obsidian, the 

 other both at once ; one furnishes only loose cinders, the 

 other lava flowing in narrow streams. These characteristic 

 processes, moreover, 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 distinc- 

 tions only occur in those groups with regard to which we 

 must confine ourselves to definite historical periods near to 

 our own times. The non-detection of single lava-streams 

 depends simultaneously upon many conditions. Amongst 

 these we may instance the deposition of vast layers of tufa, 

 rapilli, and pumice-stone ; the simultaneous and non-simul- 

 taneous confluence of several streams, forming a widely ex- 

 tended lava-field covered with detritus ; the circumstance that 

 in a wide plain the small conical eruptive-cones, the volcanic 



Elatform, as it were, from which, as at Lancerote, the lava 

 ad flowed forth in streams, have long since been destroyed. 

 In the most ancient conditions of our unequally cooling planet, 

 in the earliest foldings of its surface, it appears to me very pro- 

 bable that a frequent viscid outflow of trachytic and doleritic 

 rocks, of masses of pumice-stone or perlite, containing obsi- 

 dian took place from a composite network 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 



