358 SCIENCE PROGRESS 



justified by Harker's observations in another area within the 

 British Isles. Geologists will remember that opposition to these 

 views came from a reluctance to accept the large demands of 

 Levy, Lacroix, and Barrois in France, not to say of Hawes in 

 America in 1881. Critics still urge the greater probability of 

 the differentiation of molten magmas, as against the admixture 

 of already differentiated materials, and see considerable difficul- 

 ties in the way of producing composite rocks. F. D. Adams J 

 in Canada would, it seems, refer banded gneisses to the 

 metamorphism of sediments rather than accept as a general 

 proposition the theory of interpenetration and partial absorption. 

 Harker himself argues for an essential difference between true 

 composite rocks and rocks of composition intermediate between 

 two well-known petrographic types. The " intermediate rocks " 

 are for him, as with Brogger, representatives of a special magma, 

 and cannot be imitated by admixture. Yet the experimental 

 researches of Doelter 2 point to the unlimited possibilities of 

 admixture, provided that the temperature is sufficiently high 

 in the caldrons underground. Doelter also points out the 

 corroding effect of the gases associated with a molten and 

 invading magma 3 : "In oberen Schichten scheint die Korro- 

 sionswirkung mitunter recht gering gewesen zu sein, das wurde 

 aber nicht hindern, dasz sie in tiefen Schichten bedeutend sein 

 kann und dasz Magma sich langsam hinauffriszt, denn die Gase 

 wirken wie eine Lotrohrflamme." The gases, moreover, that 

 we know, in various combinations, among the emanations of 

 volcanos, are potent influences in promoting the crystallisation 

 underground of such minerals as quartz, mica, garnet, epidote, 

 etc. May we not be closely approaching a time when we 

 shall generally regard dynamic metamorphism, combined with 

 movement, as the destroyer of distinct crystallisation in a schist, 

 and contact-metamorphism, on a regional scale, as the main 

 promoter of such crystallisation ? Sometimes a sediment, 

 foliated by pressure, in which all trace of original bedding has 

 been lost, becomes invaded and heated, and new minerals arise 

 along the foliation-planes ; at other times a sediment is invaded 



1 "Some recent papers on the Influence of Granitic Intrusions upon the 

 Development of Crystalline Schists," Journ. of Geology, vol. v. (1897), p. 293. 



2 These researches are admirably summed up, with those of other workers, in 

 his recent book, Pedogenesis (1906). On admixture, see pp. 80, 118, etc. 



3 Ibid. p. 119. 



