AGE OF GRANITE. 79 



two igneous rocks. Their mineralogical difference is not great and their 

 chemical difference is small, but in texture they are widely contrasted. 

 The granite is coarse-grained to a degree that is exceptional even for 

 granite. The trachyte has a fine and uniform texture, and its crystals are 

 embedded in a paste. All the analogies of modern petrography lead to the 

 assumption that the conditions under which the two rocks cooled were very 

 different. The trachyte must have solidified with some degree of rapidity, 

 else its paste would not have remained. The granite, in order to have 

 separated its minerals so completely and in masses of such size, must have 

 congealed with infinite slowness. But if the granite was injected beneath 

 the Potsdam in the early part of the Tertiary period just as the trachyte 

 was injected beneath the Potsdam near the beginning of Tertiary time, the 

 conditions under which the two were consolidated could not have been 

 vastly different. 



The first conclusion having been unsettled by these considerations, the 

 question was asked whether it was not possible that peculiar atmospheric 

 conditions in Potsdam time made granite a more perishable rock than it was 

 during the Tertiary and Quaternary, and thus prevented the preservation 

 of its bowlders. An examination was made of the descriptions of the basal 

 beds of the Potsdam in the Northern States and Canada and in the Rocky 

 Mountain region, and it was found that while silicious pebbles were every- 

 where observed as predominant and foliated rocks were occasionally noted, 

 there was but a single record of granitic pebbles. It was seen that the 

 absenqe of feldspathic pebbles, from whatever cause it arose, was not pecu- 

 liar to the Black Hills locality, and the conclusion based upon this absence 

 was relinquished. 



When, therefore, in a subsequent year a feldspathic pebble was obtained 

 from the Potsdam its discovery occasioned no surprise, for it merely estab- 

 lished by positive proof a conclusion already entertained as the result of 

 indirect but cumulative evidence. 



The discovery of granitic debris in the conglomerate at the base of the 

 fossiliferous rock system fixed the age of the granite as nearly as it is likely 

 ever to be determined. It not only antedates the Primordial sediments, but 

 it antedates a great division of Archaean time. We must count as Archaean 



