136 Trachyte and Basalt. paet i. 



which consists chiefly of feldspar, with some hornblende 

 and oxide of iron, has a specific gravity of about 2*45 ; l 

 whilst basalt composed chiefly of augite and feldspar, 

 often with much iron and olivine, has a gravity of about 

 3"0. Accordingly we find, that where both trachytic 

 and basaltic streams have proceeded from the same 

 orifice, the trachytic streams have generally been first 

 erupted, owing, as we must suppose, to the molten lava of 

 this series having accumulated in the upper parts of 

 the volcanic focus. This order of eruption has been 

 observed by Beudant, Scrope, and by other authors ; 

 three instances, also, have been given in this volume. 

 As the later eruptions, however, from most volcanic 

 mountains, burst through their basal parts, owing to 

 the increased height and weight of the internal column 

 of molten rock, we see why, in most cases, only the 

 low r er flanks of the central, trachytic masses, are en- 

 veloped by basaltic streams. The separation of the 

 ingredients of a mass of lava would, perhaps, sometimes 

 take place within the body of a volcanic mountain, if 

 lofty and of great dimensions, instead of within the 

 underground focus ; in which case, trachytic streams 

 might be poured forth, almost contemporaneously, or 

 at short recurrent inter vals, from its summit, and 

 basaltic streams from its base : this seems to have taken 

 place at Teneriffe. 2 I need onh T further remark, that 

 from violent disturbances the separation of the two 

 series, even under otherwise favourable conditions, 

 would naturally often be prevented, and likewise their 



1 Trachyte from Java, was found by Von Buch to be 2 - 47 ; from 

 Auvergne, by De la Beche, it was 242 ; from Ascension, by myself. 

 it was 2-42. Jameson and other authors give to basalt a specific 

 gravity of 3*0 ; but specimens from Auvergne were found, by De la 

 Beche, to be only 2 - 78 ; and from the Giant's Causeway, to be 2-91. 



2 Consult Von Buch's well-known and admirable Description 

 Physique of this island, which might serve as a model of descriptive 

 geology. 



