76 (iEOLOCJV OF THE BLACK HILLS. 



when the surface of" the nioltcii <>]obe first cooled, and so indigenous, or 



else the result of an extrusion of the still molten interior magma, and so 



exotic. Later investigations showed that in some places granite passes by 



insensible gradations into gneiss, a rock that is universally recognized as 



metamorphic, and left no douljt that some granites are the products of tlie 



metamorphism of sedimentary rocks. A school of geologists at once arose 



who maintained that all exotic as well as indigenous granites were the 



result of the fusion or partial fusion of ancient sediments, and that the 



original crust of the earth and the uncongealed substance of the interior 



are not illustrated by the rocks accessible to the geologist. Professor llam- 



sey, an advocate of this view, says: 



Tliat in one sense it [granite] is an igneous rock ; tluit is to say, that it bas been 

 completely fused. But in anolber sense it is a nietamorpbic rock, j)artly because it is 

 impossible in many cases to draw any definite line between gneiss and granite, for 

 they pass into eacii other by in.sensible gradations; and granite occupies the space 

 that ought to be tilled with gneiss, were it not that the gneiss has been entirely fu.sed. 

 I believe, tlierefore, that granite and its allies are siniidy the efiect of the extreme of 

 mctamorpliism, brought about by great heat with presence of water. In other words, 

 when the metamorphism has been so great that all traces of the semi-crystalline lami- 

 nated structure has disappeared, a more perfect crystallization has taken place.* 



At the present time it is admitted on all hands that there are certain 

 granites which are merely highly crystalline gneisses, but there are those 

 who doubt the metamorphic origin of the exotic or intrusive granites. 



It lias already been intimated that the granite of the Black Hills is 

 intrusive, and many of the facts by which its relation is proved have been 

 narrated in describing its distribution ; but the latter will be recapitulated 

 here, in connection with all others that seem pertinent, for the sake of giving 

 completeness to the demonstration. 



First. The feldspar, amounting to 75 per cent of the granite, could 

 not have been supplied by a simple metamorpliisni of the schists, which 

 are not very generall}- gneissoid, but more commonly micaceous, and in 

 general may be said to contain 75 per cent of mica The maximum ratio 

 of alkalies in the slates and schists is only about 8 per cent, while in 

 the granite it is fully 15 per cent. 



It ma}' be said, by the way, that the granite is chemically very similar 



• Physical Geology and Geography of Great Britain. A. E. Ramsey, p. 38, 'i ed. 



