chap. vi. Trachyte and Basalt. 135 



the formation of detached granules. The specific 

 gravity of silver is 10 - 4, and of lead 11*35 : the granu- 

 lated lead, which sinks, is never absolutely pure, and 

 the residual fluid metal contains, when richest, only 

 y-fg- part of silver. As the difference in specific gravity, 

 caused by the different proportions of the two metals, 

 is so exceedingly small, the separation is probably aided 

 in a great degree by the difference in gravity between 

 the lead, when granular though still hot, and when 

 fluid. 



In a body of liquefied volcanic rock, left for some 

 time without any violent disturbance, we might expect, 

 in accordance with the above facts, that if one of the 

 constituent minerals became aggregated into crystals 

 or granules, or had been enveloped in this state from 

 some previously existing mass, such crystals or granules 

 would rise or sink, according to their specific gravity. 

 Now we have plain evidence of crystals being embedded 

 in many lavas, whilst the paste or basis has continued 

 fluid. I need only refer, as instances, to the several, 

 great, pseudo-porphyritic streams at the Galapagos 

 Islands, and to the trachytic streams in many parts of 

 the world, in which we find crystals of feldspar bent 

 and broken by the movement of the surrounding, semi- 

 fluid matter. Lavas are chiefly composed of three 

 varieties of feldspar, varying in specific gravity from 

 2 '4 to 2*74 : of hornblende and augite, varying from 3*0 

 to 3 *4; of olivine, varying from 3 "3 to 3*4; and lastly, 

 of oxides of iron, with specific gravities from 4*8 to 5*2. 

 Hence crystals of feldspar, enveloped in a rows of 

 liquefied, but not highly vesicular lava, would tend to 

 rise to the upper parts ; and crystals or granules of the 

 other minerals, thus enveloped, would tend to sink. 

 We ought not, however, to expect any perfect degree 

 of separation in such viscid materials. Trachyte, 



