FUSION AND ABSORPTION. 733 



contact between the quartz-keratophyre and gabbro, and has an even greater 

 width, averaging perhaps 90 meters, and being in some places as wide as 

 150 meters. But only a comparatively small part of this belt, that nearest 

 the rhyolite, has the textures and structures of an igneous rock, and seems 

 to have been completely fused and recrystallized into slates and quartzites. 

 The fusion metamorphism produced by the metarhyolite is correlated with 

 the enormous masses of intrusive granite which occur in the Keweenawan 

 series above the Animikie. The Pigeon Point dike of this rock doubtless 

 is located at one of the passages through which the material of some of 

 these larger masses made their way toward the surface. In this connection 

 it is notable that subsequent intrusive trap dikes cut all of the other rocks 

 of Pigeon Point with sharp contacts, and no sign of metamorphosing effects- 

 Bayley states that the metarhyolite itself may be the product of the 

 action of the gabbro upon the slates and quartzites. If this suggestion 

 is correct the Pigeon Point locality is a still more remarkable case of the 

 production of a rock by fusion of sediments. But that the rhyolite has 

 been produced from the slates can not be considered as determined. Indeed, 

 the analyses of the rhyolites and of the slates and quartzites are different 

 from each other in various important respects. While the amount of silica 

 and alumina in the two rocks is substantially the same, the alkalies are 

 nearly twice as abundant in the rhyolite as in the sedimentary rocks. Also, 

 the iron is more than twice as abundant in the sedimentary rocks as in the 

 igneous rocks. Nor can it be supposed that the surplus of alkalies and the 

 deficiency of iron in the rhyolite can be explained by the gabbro, it modi- 

 fying the composition of the fused slates and quartzites so as to produce a 

 magma of the composition of the rhyolite; for if any considerable portion 

 of g-abbro magma had been added the silica content would have fallen and 

 the alumina would have gone up. Furthermore, the gabbro is scarcely 

 richer in alkalies than the slates and quartzites; hence, on the theory that 

 the red rock was formed by the fusion of the slate and quartzite by the 

 gabbro, one would have to explain why the quantity of alkalies in the red 

 rock is about twice as great as in either of its possible sources. Thus the 

 analyses seem to stand in the way of the conclusion that the granular red 

 rock (metarhyolite) is really a result of the fusion of the Animikie slates 

 and quartzites by the gabbro. a 



a Clarke, F. W., and Hillebrand, W. F., Analyses of rocks and analytical methods: Bull. U. S. 

 Geol. Survey No. 148, 1897, pp. 106-110. 



