W. Cross — Denver Tertiary Formation. 271 



era! substances composing the Denver beds having been stated, 

 it remains to consider their origin in order that the full value 

 of this evidence may be appreciated. Attention is first called 

 to the eruptive rocks, as the predominant material. 



The first point in evidence is negative. There is no known 

 source which can be assigned with plausibility for any one of 

 the many anclesitic types represented in the Denver strata. No 

 andesite masses are known in the mountainous area to the 

 westward as far as the continental divide, though it is admitted 

 that they may exist in local development. The more distant 

 andesitic masses in Middle or South Park cannot be considered, 

 nor a hypothetical transient volcanic vent in the plains area, 

 for neither of these explanations can meet the facts of observa- 

 tion. The problem is a double one, viz : to account for the 

 exclusion of the common materials (quartz, etc.) simultaneously 

 with the appearance of the unusual, and this in a basin adjacent 

 to a mountainous Archaean district. 



These considerations have determined the form of the solu- 

 tion to be offered. The andesitic masses which furnished the 

 materials for the lower part of the Denver sediments were so 

 situated as to effectually prevent the access of all Archaean and 

 sedimentary debris to the lake of that epoch. That is to say, in 

 the interval between the Willow Creek and Denver epochs 

 there was an outpouring of andesitic lavas completely covering 

 the Archsean and sedimentary rocks of the area afterwards con- 

 tiguous to the Denver lake. When sedimentation began again 

 only eruptive debris could appear in the deposits until erosion 

 and general degradation had laid bare, here and there, small 

 areas of granite, of gneiss or of sandstone. 



The Denver strata contain the record of the destruction of 

 a great series of allied lavas. The nine hundred feet of fine- 

 grained sediments represent a vastly greater amount of rock 

 destroyed, and in the series of coarse bowlder beds is evidence 

 of the practical completion of this work. Then came a return 

 to the surface conditions existing during the Willow Creek 

 epoch. 



The other materials of the Denver beds are easily accounted 

 for. In the coarse bowlder beds of Green Mountain both 

 Archaean and earlier sedimentary rocks are very prominent. 

 They were undoubtedly derived from the adjacent western 

 shore after the andesitic covering had been worn through. 



A different origin is indicated for the quartz and red feld- 

 spar material in the fine-grained strata of the plains. Experi- 

 ence shows these substances to be very local in development 

 and to be most abundant adjoining the northern and southern 

 Willow Creek shore-lines, which were in great degree made up 

 of friable or soft grits and sandstones. The absence of the 



