W. Cros» — Denver Tertiary Formation. 269 



a case parallel to that of the occurrence of Silurian pebbles in 

 the Dakota. Like the pebbles of the Dakota, too, only in a far 

 greater degree, those of this formation have undergone extreme 

 silicification. Jaspers, agates, flints and silicified wood abound, 

 and the debris of the older groups, including the fossiliferous 

 limestone just noted, has often undergone the most complete 

 alteration in this manner. This feature, however, is especially 

 noticeable only in that portion of the formation which, from 

 having been laid down within a comparatively short distance 

 of what was probably the ancient shore line, contains a very 

 large amount of pebbles of the older rocks, and is thus best 

 calculated to show any changes of this kind that may have 

 occurred." 



Relation of the Denver to the Willow Creek. — The Denver 

 beds occupy a basin eroded out of the Willow Creek, in the 

 greater part of the area now under discussion. There is thus 

 a general non-conformity between the two deposits, while the 

 details of this relationship may be seen in certain places. 



Though the actual contact line of the two Formations is sel- 

 dom seen, owing to the friable nature of the strata and the 

 prevalence of surface deposits, there is no difficulty in assign- 

 ing isolated outcrops to the proper series, on lithological 

 grounds which have been explained. The interval between 

 the Denver and Willow Creek epochs will be spoken of in a 

 succeeding section. 



Relation of the Denver to the Monument Creek. — As has 

 beeu stated in the descriptive part, about two thirds of the 

 known thickness of the Denver beds were removed prior to the 

 deposition of the Monument Creek Formation. There is, 

 moreover, no reason to suppose that the upper strata of Green 

 Mountain represent the actual top of the Denver beds, nor can 

 we now determine the former lateral extent of the Formation. 



Concerning the length of the time interval between Denver 

 and Monument Creek deposits we have no data. 



b. Lithological evidence. 



The materials of the Denver beds. — It is a fact of general 

 experience that quartz is usually the most abundant mineral in 

 strata which are made up of the worn debris of older rocks. 

 This position comes to the quartz largely by reason of its 

 superior hardness and the absence of cleavage, properties enab- 

 ling it to resist attrition better than the other common rOck- 

 making minerals. When detritus from areas of the crystalline 

 schists has been the chief element in making up the sediments 

 of adjacent oceans or seas, the relative amount of quartz in the 

 strata formed will depend upon the violence of the destructive 



