58 Washington — Study of the Glaucophane Schists. 



Finally Barrois* regards the glaucophane schists of lie de 

 Groix, in Brittany, as the product of the metamorphism of 

 sedimentary rocks. 



Compariso?i with amphibolites. — A point of some interest, 

 already touched on by Rosenbusch, f is the general similarity in 

 chemical composition between the basic glaucophane schists 

 and many of the amphibolites, as may be seen on reference to 

 the analyses of these last given by Roth, Zirkel and Rosen- 

 busch. It was hoped that the present investigation would 

 throw some light on this point, and possibly reveal some con- 

 stant difference in chemical composition between the two, 

 which would serve to explain the diverse mineralogic resultants 

 of apparently identical metamorphic processes on similar 

 original material. 



The results are, on the whole, inconclusive, though as a rule, 

 the amphibolites are higher in MgO, CaO and K 2 0, and lower 

 in Fe 2 3 , than the glaucophane schists. The eclogites approach, 

 as a class, much more closely to the glaucophane schists, espe- 

 cially in low K 2 0, though they also are apt to be higher in 

 MgO and lower in Fe 2 3 . 



As Rosenbusch remarks, the distinction between the amphi- 

 bolites and the glaucophane schist lies in the fact that, while in 

 the former the Na 2 has gone into feldspar, in the latter it has 

 gone into glaucophane. He suggests that this may be con- 

 nected with a difference in the age of the rocks. 



With this view I cannot agree. It seems more reasonable 

 to suppose that, just as in the consolidation of igneous magmas 

 the eventual mineralogic composition of rocks derived from 

 any given magma is chiefly dependent on the physical condi- 

 tions of cooling, the presence of mineralizers, etc., so here 

 physical or chemical conditions have determined whether the 

 metamorphism of, for instance, a diabase tuff produces a 

 normal amphibolite or an epidote- glaucophane schist. 



This view is based partly on the general reasoning which has 

 abolished the age distinction in igneous rocks, and partly on 

 the following considerations : 



In the first place, we find at many places, as lie de Groix, 

 Greece, California, Anglesey, etc., amphibolites and glauco- 

 phane schists occurring together, the latter being often only 

 locally developed. 



In the next place glaucophane has been often developed at 

 one locality by the metamorphism of such widely diverse 

 original materials as cherts and diabase tuffs. This points 

 clearly to the existence of some peculiar conditions, apart from 



p. 72. 



*Barrois, Eef. Neus Jahrb., 1884, ii, 

 | Rosenbusch, op. cit., p. 711. 



