208 CLARENCE KING'S CLASSIFICATION OF LAVAS. 



compared, there are no such, differences between them as would suggest 

 difficulties, on that account, in recognising their homologues among 

 stratified deposits. This hypothesis shows how the microscopic 

 crystalline texture may help to unfold the evolution of lava, and yet 

 how subordinate from a physical point of view are the refinements of 

 such analysis, since they manifest what may be termed mechanical 

 accidents which have governed the development of certain minerals in 

 natural association in igneous rocks. 



Clarence King's Views on American Volcanic Rocks. Mr. 

 Clarence King conceives the volcanic rocks to belong to an altogether 

 different type from the plutonic rocks, and to have originated in 

 different ways. He bases his distinction upon the occurrence of 

 glass inclusions in the crystals of lavas, and fluid inclusions in the 

 crystals of plutonic rocks. To us this difference is chiefly indicative 

 of the different conditions of pressure under which the rocks respec- 

 tively cooled, which allowed the water of volcanic rocks to escape. 

 It is, moreover, by no means a universal law, and the fact that one 

 whole class of volcanic rocks, the propylites, is characterised by fluid 

 inclusions in the quartz, prevents us from attaching fundamental 

 importance to this condition. And in consequence of this circum- 

 stance, Mr. Clarence King, who finds no difficulty in accepting the 

 metamorphic origin of granites, does not see his way to accept the 

 metamorphic origin of lavas. Further, he finds each of the types of 

 lava, in the far west of the United States, to present such mineral 

 differences that each may, as a rule, be said to present three modifi- 

 cations, characterised respectively by quartz, hornblende, and augite, 

 as in the following scheme: 



Clarence King's Classification of Tertiary Volcanic Rocks of America. 



!Quartz-Propylite. 

 Hornblende-Propylite (rarely micaceous). 

 Augite-Propylite. 

 SQuartz-Andesite (or Dacite). 

 Hornblende- A ndesite (rarely micaceous). 

 Augite- An desite. 

 I Quartz-Trachyte. 



Trachyte . . j Mica-Trachyte (rarely hornblendic) . 

 ( Augite-Trachyte. 



iQuartz-Rhyolite (or nevadite). 

 Mica-Rhyolite (rarely hornblendic). 

 Basalt. 



The propylite and andesite are regarded as of pre-miocene age, the 

 trachyte, and so-called neolite, are classed as post-miocene. Each of 

 these groups is supposed to have been erupted in succession from 

 subterranean lakes in which fusion was produced as a consequence 

 of denudation, which removed the vertical pressure of superincumbent 

 rock, and so enabled the heated mass to become liquid. It is presumed 

 that with successive ages of geological time denudation increased, and 

 fusion extended deeper and deeper. We have failed to discover any 

 evidence of such successive denudations, though the denudation of 

 the great American table- land was beyond doubt enormous ; and the 



