BLACK HILLS GRANITE EXOTIC. 77 



to the volcanic rock in the northern part of the Hills, and may perhaps be 

 closely related in origin. Only a difference in the conditions of cooling 

 would be needed to produce from the same magma a granite on the one 

 case and a feldspathic volcanic rock on the other. 



Second. The occurrence of pieces of schist rock inclosed in the mass 

 of the granite is evidence of a plastic condition of the granite coincident 

 with a non-plastic condition of the schist. 



Third. Each granite mass has a long lenticular shape, and several of 

 them could sometimes be traced in a series along the same line of bedding. 

 This is the character which veins following the strata of a pliant rock like 

 mica schist are most apt to assume. 



Fourth. The coarseness of the crystallization, the thoroughness of the 

 separation of the several minerals, and the occasional concentric arrange- 

 ment of the minerals on a large scale, all indicate a condition of the mass 

 in which there was great freedom of movement among the particles — a 

 condition of plasticity if not of perfect fluidity. 



Fifth. The transition from the schist to the granite is always sudden ; 

 no graded passage of one rock to the other was anywhere observed. 



The problem of the age of the granite occupied our attention from the 

 time of our first examination, and for a while the weight of evidence led 

 to a conclusion that is now demonstrably erroneous. The history of the 

 problem is instructive as an illustration of the danger inherent in negative 

 evidence. 



The granite was intruded between strata of the Archaean; therefore 

 it is younger than those strata. Its fragments were found as pebbles of 

 the conglomerate at the base of the White River Tertiary; therefore it is 

 older than that formation. There was no considerable disturbance during 

 the deposition of the strata from the Potsdam to the Cretaceous, inclusive; 

 therefore the intrusion did not take place in Paleozoic or Mesozoic time. 

 The granite is coarsely and completely crystalline; therefore it was cooled 

 slowly and under pressure, and it could not have been intruded after the 

 Archaean strata were uplifted and eroded and before the}^ were covered by 

 the Potsdam. There remain two possible dates or epochs, the first in 



