234 Clarence King. 



been established by more recent work, especially in the domain 

 of microscopic petrography, lie showed such grasp of his sub- 

 jects, and such originality and power of thought, that his views 

 constituted not only an important advance over those of the day, 

 but they were suggestive of the lines of investigation that have 

 been most fruitful in the modern advance of geological science. 



For instance, in his discussion of the reason for the changes 

 from acid to basic eruptives within the individual groups, which 

 he proposed as a variation from the natural order in age of 

 volcanic rocks, as laid down by Richthofen, he advanced 

 views very suggestive of the modern conception of differentia- 

 tion in eruptive magmas. 



Again, in endeavoring to account for the formation of those 

 types of granite that pass into gneiss and crystalline schists of 

 essentially the same chemical composition, but which show no 

 evidence of having been subjected to such excessive heat as 

 would produce actual liquefaction, he called in the agency of 

 the immense pressure to which such rocks would necessarily 

 have been subjected. While the long years of combined field 

 work and microscopic study of modern petrographers, made 

 since King's theory was enunciated, have proved that the 

 structure of crystalline schists is due to pressure, they do not 

 go so far as he did in assuming that the end product of such 

 mechanical pressure might be granite. 



Perhaps his most enduring theoretical discussion of that time 

 was that on hypogeal fusion, in which, accepting the validity 

 of the physical arguments against the fluid interior of the earth, 

 he discusses and rejects Hopkins' theory of residual lakes and 

 Mallett's conception of local lakes produced by mechanical 

 crushing. He then advances an hypothesis of his own which 

 may be called that of a critical shell, or couche, between the 

 permanently solid interior and the outer crust of the earth, 

 which is above the temperature of fusion but restrained from 

 fusion by pressure. In this, therefore, the opposing forces of 

 pressure and temperature hold themselves reciprocally in equi- 

 librium, but when this equilibrium is disturbed, as for instance, 

 by a sudden change of the relative position of isobars and 

 isotherms — say by local erosion and rapid transfer of load 

 within limited areas — local lakes of fusion would be created. 

 Iddings, in his " Origin of Igneous Rocks," says of King's 

 treatment of this subject: "By the breadth of his treatment 

 and by better and fuller data he advanced the problem of the 

 origin of the various kinds of volcanic rocks far beyond the 

 point reached by any of his predecessors." 



In his chapter on Orography, King says, in speaking of the 

 causes of crust motion : " I can plainly see that were the criti- 

 cal shell established its reactions might thread the tangled 



