78 GEOLOGY OF THE BLACK HILLS. 



Archaean time before the processes of displacement and erosion had brought 

 the inclosing- schists near to the surface, the second in Tertiary time before 

 the deposition of the White River beds. 



This much could be deduced from the positive data of observation 

 taken in connection with generally accepted principles; but the next step 

 was taken on the authority of negative evidence. No granite whatever 

 was found in the conglomerate at the base of the Potsdam sandstone, and 

 this although definite search was made at every opportunity. The Potsdam 

 was found to contain a great quantity of quartzitic pebbles easily referable 

 to the Archaean quartzites and occasional specimens of Archaean schists, 

 but no granite. This conspicuous absence, coupled with the presence of 

 granite in the Miocene conglomerate and Quaternary gravels seemed to say 

 that the granite was not a member of the subjacent rock system when the 

 Potsdam was laid down and pointed to Post-Cretaceous time as the date of 

 its introduction. 



The conclusion thus reached was opposed by the analogy of other 

 regions, but it was the logical sequent of the premises if weight was to be 

 given to the negative element, and it was provisionally entertained. Its 

 hold was soon weakened, however, when attention was directed to certain 

 other facts of observation, the importance of which was not at first appre- 

 ciated. As already described, a locality had been seen where a continuous 

 sheet of the Potsdam passes from a surface of eroded schists to a surface of 

 granite. There was found no intrusion of the granite along the parting 

 between the Potsdam and the schists, and there was found no metamorphism 

 of the Potsdam at the surface of contact with the granite. In these par- 

 ticulars the relations of the granite are strongly contrasted with those of 

 the trachyte of the Hills. Wherever the trachyte appears beneath the 

 Potsdam the latter is uplifted as though by the insertion of the trachyte 

 between it and the Archaean, and its lowest beds are at the same time 

 metamorphized as though by the heat of the molten intrusion. The fact 

 that the granite did not at this locality affect the form and constitution of 

 the Potsdam strata in a manner similar to the trachyte does not well accord 

 with the idea that it was introduced under similar conditions and during 

 the same geological period. 



To the same intent are the minera logical and textural relations of the 



