W. Cross — Denver Tertiary Formation. 267 



mation. A study of numerous exposures shows that the remark- 

 able freedom from non-eruptive materials characterizing the 

 Table Mountain beds does not strictly hold for these equivalent 

 strata. Along the Platte River and to the eastward one can 

 generally detect a small amount of quartz or of red feldspar 

 (microcline) in the more sandy Denver beds, though these sub- 

 stances are lacking in many places, even along Coal Creek, 

 farthest from the foothills. Quartz and red microcline are 

 taken as representing Archaean rocks, directly or indirectly 

 They become prominent in the Denver beds of the plains area 

 only locally and under circumstances which will be considered 

 in another part of this article. As a result of their composi- 

 tion the finer-grained Denver rocks are easily recognizable 

 when one is at all familiar with them. They possess a dull 

 reddish brown color, and while friable and crumbling they re- 

 sist degrading agencies in a manner peculiar to them. This 

 is due to the admixture of clay with most sands, and to the 

 development of a zeolitic cementing substance derived from 

 the andesite. Tests show that as high as 50 per cent, of some 

 sandy strata are soluble in hydrochloric acid with production 

 of gelatinous silica. 



Fossils. — The fossil flora of Table Mountain has been re- 

 ferred to. It is one of the richest yet discovered and has been 

 very fully described. As one of the earliest discoveries in the 

 west it has been an important element in discussing the floras 

 found more recently in other formations. During the present 

 work fossil plants were observed in many places and some col- 

 lections were made, but so far few species have been found which 

 were not already known in Table Mountain. Good localities 

 might be developed in various places over the entire field. 



A very few invertebrate fossils have been found in plant- 

 bearing Den ^er beds, but they are not of much determinative 

 value. 



By far the most important, as well as the most interesting 

 fossils of the Denver beds are the large bones found in several 

 places, which are provisionally referred by Prof. O. 0. Marsh 

 to various Cretaceous types of the Dinosauria. A single 

 fossil from the west bank of the Platte River, near Denver, 

 has been described by Prof. Marsh as a new species of Bison, 

 and a probable Pliocene age ascribed to the beds containing it. 



A consideration of the evidence afforded by these various 

 groups of fossils as bearing upon the age of the Denver For- 

 mation shows that these different elements are in conflict with 

 each other and with the stratigraphy. This fact renders nec- 

 essary an examination as to the relative values to be given to 

 these conflicting evidences. Some results of this examination 

 will be given in the succeeding sections of this article. 



