NO IGNEOUS OVERFLOWS. 191 



important than the differences, and leave little room to doubt that the peaks 

 are to be classed tog-ether as the result of a single group of" conditions and 

 the product of a single period of eruption. 



In the structure of the peaks the first fact that strikes the observer is 

 their uniformly conical shape. To this rule Bear Lodge is the only excep- 

 tion, and this, with its symmetrical colunniar shape, is the most remarkable 

 of all. The peaks appear to be merely pointed or conical waves of igneous 

 rock forced upward through the sedimentary strata, which are found dis- 

 turbed and turned up around them only in their immediate vicinity. The 

 metamorphism of tlie upturned strata is limited in extent, reaching onl}- a 

 few feet from their contact with the igneous rocks. There was observed no 

 evidence of any ovei'flow of the Igneous matter,* but it is confined exclu- 

 sively to the cores of the peaks. The view that they are the cores of extinct 

 volcanoes or centers of igneous overflow is scarcely warranted by the 

 observed facts. It would be expected, if such were the case, that the sedi- 

 mentary rocks tilted around them would have been more metamorphosed 

 by the igneous heat, and even though a large portion of the overflowing 

 material was removed by denudation, it is scarcely possible that every indi- 

 cation of such an overflow could have been so completely destroyed. It 

 would appear therefore that the igneous peaks, instead of being the product 

 of violent volcanic action, are situated at a great distance from the central 

 and maximum region of igneous action; and that instead of the material 

 being ejected with great violence and at such a temperature as to cause it 

 to overflow readily, it was forced upward through the sedimentary strata 

 under great pressure and at such a temperature as to make it plastic rather 

 than fluid. The occurrence of these trachytic peaks appears like a great 

 pustular outbreak on the surface of the northern end of the Hills whereby 

 the deep-seated igneous forces were relieved, or like the appearance of 

 bubbles on the surface of a kettle of boiling tar. 



The peaked character of the igneous points seems to be somewhat 

 peculiar, for in the records of geological explorations in the Rocky Mount- 

 ains it is not mentioned. Mr. Dawson, however, in his report on the region 



* Possibly au exception to this statement should he made, for, as related in the (h'scription of 

 TeiTy Peak, rhyolite near the Deer Mountains was seen overlying quartzite helougiug probably to the 

 Potsdam. 



