IGNEOUS EPOCH. , 219 



This last conclusion has an important bearing in another direction. 

 The general degradation of the Hills has made little progress since the 

 time of the Miocene conglomerate. Some drainage line or lines had then 

 cut so deeply as to touch the granite, and at the present time the bottom of 

 the deepest valley in the same vicinity is only 2,000 feet lower. Two 

 thousand feet would be an incredibly small amount of erosion in a mount- 

 ain district for the indicated interval of time if the conditions of erosion 

 had always remained as they are now. But the difficulty lessens when it 

 is known that the erosion was retarded first by a long lake period, during 

 which the water-ways of the Hills were in part clogged by their own debris, 

 and second by a long period of slow general degradation, during which the 

 accumulated lake sediments were little by little earned away and the 

 "base level of erosion" was slowly depressed. 



4. — The age of the igneous rocks. 



The igneous masses have so much in common, both in the character of 

 their material and in their relation to the stratified rocks, that they are 

 assumed to have a common epoch of formation, and whatever is determined 

 with reference to one is by analogy ascribed to the whole. Unfortunately 

 our evidence is not at all precise, and serves only to indicate certain widely 

 separated time limits, between which the bosses of trachyte and rhyolite 

 were formed. 



In nearly every locality of igneous rock the adjacent sedimentary 

 rocks were found to have undergone some modification either of character 

 or of position. Wherever a clay or sandstone was seen in contact with the 

 intrusive mass it was found to have been more or less metamorphosed by the 

 original heat ; and about many of the peaks the strata are upturned at con- 

 siderable angles in such way as to leave no question that their flexure is a 

 phenomenon of the volcanic event. In all such cases it is evident that the 

 modified strata are older than the associated intrusives. In different places 

 all members of the sedimentary series from Potsdam to Dakota were 

 found upturned around igneous centers, and at a single locality, Bear 

 Butte, the same influence was found to extend to the Fort Benton group, 

 No. 2 of the Cretaceous. The higher members of the Cretaceous and the 



