76 GEOLOGY OF THE BLACK HILLS. 



when the surface of the molten globe 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 doubt that some granites are the products of the 

 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 Ram- 

 sey, an advocate of this view, says: 



That in one sense it [granite] is an igneous rock ; that is to say, that it has been 

 completely fused. But in another sense it is a metamorphic rock, partly because it is 

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

 they pass into each other by insensible gradations ; and granite occupies the space 

 that ought to be filled with gneiss, were it not that the gneiss has been entirely fused. 

 I believe, therefore, that granite and its allies are simply the effect of the extreme of 

 metamorphism, 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 merety highly crystalline gneisses, but there are those 

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



It has 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 nietamorphism of the schists, which 

 are not very generally 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 may be said, by the way, that the granite is chemically very similar 



* Physical Geology and Geography of Great Britain. A. E. Ramsey, p. 38, 2 ed. 



