chap. vi. Trachyte and Basalt. 133 



the crystals of feldspar become more and more numerous, 

 as the depth or thickness increases, so that near the 

 lower surface of the stream the lava even resembles a 

 primary rock. Yon Buch further states, that M. Dree, 

 in his experiments in melting lava, found that the 

 crystals of feldspar always tended to precipitate them- 

 selves to the bottom of the crucible. In these cases, I 

 presume there can be no doubt l that the crystals sink 

 from their weight. The specific gravity of feldspar 

 varies 2 from 2*4 to 2*58, whilst obsidian seems com- 

 monly to be from 2*3 to 2*4 ; and in a fluidified state 

 its specific gravity would probably be less, which would 

 facilitate the sinking of the crystals of feldspar. At 

 James Island, the crystals of albite, although no doubt 

 of less weight than the gray basalt, in the parts where 

 compact, might easily be of greater specific gravity 

 than the scoriaceous mass, formed of melted lava and 

 bubbles of heated gas. 



The sinking of crystals through a viscid substance 

 like molten rock, as is unequivocally shown to have 

 been the case in the experiments of M. Dree, is worthy 

 of further consideration, as throwing light on the sepa- 

 ration of the trachytic and basaltic series of lavas. 

 Mr. P. Scrope has speculated on this subject ; but he 



1 In a mass of molten iron, it is found (' Edinburgh New Philoso- 

 phical Journal,' vol. xxiv. p. 66) that the substances, which have a 

 closer affinity for oxygen than iron has, rise from the interior of 

 the mass to the surface. But a similar cause can hardly apply to 

 the separation of the crystals of these lava streams. The cooling of 

 the surface of lava seems, in some cases, to have affected its compo- 

 sition ; for Dufrenoy (' Mem. pour servir,' torn. iv. p. 271) found that 

 the interior parts of a stream near Naples contained two-thirds of a 

 mineral which was acted on by acids, whilst the surface consisted 

 chiefly of a mineral unattackable by acids. 



2 I have taken the specific gravities of the simple minerals from 

 Von Kobell, one of the latest and best authorities, and of the rocks 

 from various authorities. Obsidian, according to Phillips, is 2*35 ; 

 and Jameson says it never exceeds 2-4 ; but a specimen from 

 Ascension, weighed by myself, was 2 - 42. 



