138 Trachyte and Basalt. pakt t. 



In the second place, the perfect tranquillity, under 

 which it is probable that the plutonic masses, buried at 

 profound depths, have cooled, would, most likely, be 

 highly unfavourable to the separation of their consti- 

 tuent minerals ; for, if the attractive force, which during 

 the progressive cooling draws together the molecules of 

 the different minerals, has power sufficient to keep 

 them together, the friction between such half-formed 

 crystals or pasty globules would effectually prevent the 

 heavier ones from sinking, or the lighter oues from 

 rising. On the other hand, a small amount of disturb- 

 ance, which would probably occur in most volcanic 

 foci, and which we have seen does not prevent the 

 separation of granules of lead from a mixture of molten 

 lead and silver, or crystals of feldspar from streams of 

 lava, by breaking and dissolving the less perfectly 

 formed globules, would permit the more perfect and 

 therefore unbroken crystals, to sink or rise, according 

 to their specific gravity. 



Although in plutonic rocks two distinct species, 

 corresponding to the trachytic and basaltic series, do 

 not exist, I much suspect, that a certain amount of 

 separation of their constituent parts has often taken 

 place. I suspect this from having observed how fre- 

 quently dikes of greenstone and basalt intersect widely 



at the same time with a more refractory substance. Phonolite, as far 

 as my observations serve, in every instance appears to be an injected 

 rock, like those of the plutonic series ; hence probably, like these 

 latter, it has generally been cooled without repeated and violent dis- 

 turbances. Those geologists who have doubted whether granite 

 could have been formed by igneous liquefaction, because minerals 

 of dirf erent degrees of fusibility impress each other with their forms, 

 could not have been aware of the fact of crystallised hornblende 

 penetrating phonolite, a. rock undoubtedly of igneous origin. The 

 viscidity, which it is now known, that both feldspar and quartz retain 

 at a temperature much below their points of fusion, easily explains 

 their mutual impressment. Consult on this subject Mr. Horner's 

 paper on Bonn, ' G-eolog. Transact.' voL iv. p. 439 ; and • L'lnstitut,' 

 with respect to quartz, 1839, p. 161. 



