SOURCE OF DENVER MATERIAL. 205 



is to be mentioned that in the neighborhood oi Idaho Springs, Georgetown, 

 Empire, Central ( !ity, and other points on the eastern slope of the mountains, 

 there are numerous dikes and massive bodies of igneous rocks which, while 

 of granular or porphyritic structure, and usually coarsely crystalline, are 

 in chemical and mineralogical composition very similar to some types of 

 the andesites found in the Denver beds. They exhibit the characteristics 

 of intrusive or deep-seated eruptives, which cooled far from the surface, and 

 none of them, so far as known, can be thought of as a surface flow. 



The rocks of these bodies were first known to the writer from their 

 appearance in the bowlder beds of Clear Creek; subsequently through a 

 few specimens kindly furnished by Mr. J. S. Randall, of Georgetown, and, 

 finally, through personal visits to the localities mentioned. The rocks of the 

 Clear < 'reek bowlder beds do not come from the foothills, for no such rocks 

 are known in them, and Hear Creek, Ralston ('reek, and other streams 

 whose sources are in the foothills, do not show such rocks in the material 

 which they bring down. The direct equivalents of many of them occur 

 as dikes and stocks near Georgetown and Empire. 



It is entirely in accord with the views of the writer to suppose that the 

 diorites and diorite-porphyries of these dikes and irregular intrusive masses 

 are the deep-seated equivalents of the surface Hows represented as andesites 

 in the Denver conglomerates. It is also true that the great outpouring of 

 various lavas which must lie assumed according to the foregoing considera- 

 tions, necessitates an assumption that the channels through which the 

 molten magma ascended still exist somewhere. But there is as yet no 

 positive evidence connecting the dikes and irregular masses referred to 

 with the surface Hows assumed. The establishment of such a connection 

 will carry with it a complete continuation of the opinions which have been 

 expressed as to the geological importance of the lithological characteristics 

 of the Denver beds. Hence, while the possibility of this connection must 

 not he overlooked in judging the opinions which have Keen expressed, no 

 attempt will he made to elaborate this theoretical defense of the deductions 

 which the facts are believed to warrant. 



The western land masses adjacent to the Denver sea have alone been 

 directly considered in the foregoing remarks. The only sections «,f any 



