306 



THE EVOLUTION OF PETROLOGICAL IDEAS. 



satisfactorih^ answered. I was fortunatel}^ able to test the theory 

 quantitatively^ for Mr. Waller had already analyzed one of the quartz- 

 felsites, and Dr. Petersen had published analyses of the glass v base of 

 one of the andesites and of the devitriiied base of another. On com- 

 paring the mean of the two analyses of the base with the analysis of the 

 quartz-felsite, it was found that of the eight constituents six differed 

 by less than 0.4 per cent, silica differed by 2.2, and soda by 1.46." 



I=Glassy base of hypersthene-andesite from Fairhaugli, Usway Burn, Cheviots. (Ebcrt.) 

 II=Devitrified bast of andesite, 2 miles up AUerhope Bum. (Wulf.) 

 III=Mean of the two analyses. 

 IV=Quartz-felsite from dike on the Coquet, ont'-lialf mile alMiveSliillmore farm, Cheviots. (Waller.) 



Differentiation dependent on crystallization is a fact which can not 

 be denied; for the igneous magma, except when it cools as a gla.ss, 

 separates into distinct minerals which do not, as a rule, consolidate 

 simultaneously. But the acceptance of this fact does not involve the 

 acceptance of the differentiation theory of the origin of petrographical 

 species, for, as M. Michel Levy points out, the crystallization of a 

 magma under ordinary circumstances does not commence until it has 

 reached a pasty state. MM. Fouque and Levy observed no tendency to 

 differentiation, of the kind required to produce petrographical species, 

 in their celebrated synthetical experiments. The centers of crystalli- 

 zation were uniformly distributed throughout the masses, which were 

 too viscous to allow of any apprecial)le movement of the first-formed 

 minerals. Nevertheless, the facts observed by Darwin and others 

 clearly prove that in large masses of lava, even at the surface of the 

 earth, movement of crystaL> is possible in igneous magmas, and M. 

 Michel Levy himself admits that sucii movement may becomt> an 

 important factor under certain circumstances. 



Mr. llarker has suggested another way in which crystallization may 

 operate so as to produce variation in a mass of rock. He has shown 

 that the Cari'ock Fell gabbro varies in conquxsition from the center to 

 the sides, and that, as so frequently happens in eruptive masses, the 

 latter are more basic than theforniei-. He considers "that the differ- 

 entiation took i)lace by diffusion m a tluid magma, but not as a process 

 distinct from and quite anterior to crystallization. It was, as I believe, 

 effected in a quasi-saturated magma, concuirently with the crystalliza- 



'^ A.S the figures were not j)laeed side by side in tlie original i)ai>er (Cieol. Mag., 1885, 

 p. 106), 1 so place them now. 



