56 Washington — /Study of the Glaucophane Schists. 



Comparisons with other rocks. — Bosenbusch* has already 

 called attention to the fact that "certain glaucophane rocks 

 are chemically identical with the igneous rocks belonging to 

 the gabbro magmas, or their tuffs." He bases this conclusion 

 on the two analyses of Melville (Nos. V and X), admitting 

 at the same time that it does not follow that all glaucophane 

 rocks are so derived. 



That this main conclusion is correct for the majority of 

 glaucophane schists would seem to be borne out by the analyses 

 in the table, and their comparison with typical analyses of dia- 

 base and gabbros quoted in the large works of Roth, Zirkel 

 and Rosenbusch. It is undeniable that there is a remarkably 

 close agreement between the two, and the conclusion is irre- 

 sistible that a large part of the glaucophane schists are prob- 

 ably derived from diabases, gabbros or their tuffs. 



That this is not true of all is rendered certain by the four 

 analyses of acid' glaucophane schists, which we have already 

 seen are in all probability derived from cherts, quartzose shales 

 or quartzites. The anomalous analysis No. XI corresponds 

 with those of many rather acid diorites, and it is possible that 

 it has been derived from a rock of this kind, though this is not 

 certain. 



Although outside the scope of this paper it will not be amiss 

 to call attention to several cases which bear directly on the 

 question of the derivation of some of the glaucophane schists 

 from rocks of the gabbro family, and which are strongly con- 

 firmatory of this view. 



Direct evidence of the change from undoubted igneous rocks 

 was first given by Kotof in the case of several Japanese occur- 

 rences, melaphyrs and diabase tuffs showing a gradual increase 

 in the blue amphibole and final transition into true glauco- 

 phane schist, through " glaucophanization " of the diallage. 

 Instances of the same thing are also mentioned by Harada.J 



Another instance is furnished in Greece by Lepsius, in his 

 description of the metamorphic schist of Attica. § He shows 

 that some of the gabbros of this region, which have been 

 intruded into the sedimentaries and have been subjected to the 

 same regional metamorphism, contain glaucophane, often in 

 large quantities, though there are apparently few instances of 

 the final conversion into true glaucophane schist. 



It may be noted that the gabbros from near the Isthmus of 

 Corinth and from Argolis, according to Lepsius and my own 

 observations, do not show any glaucophane, but are analogous 



* Rosenbusch. op. cit., p. 111. 



f Koto op. cit. 



% Harada. op. cit, p. 62. 



§ Lepsius, Geologie von Attika. Berlin, 1893, p. 176. 



