272 G. B. RICHARDSON THE MONUMENT CREEK GROUP 



There are abrupt changes in composition and texture of the rocks and a 

 number of local unconformities are exposed. One of unusual promi- 

 nence occurs near the top of several buttes in the vicinity of Larkspur 

 and Greenland. Another unconformity, exposed on Dawson Butte, is 

 at the base of the rhyolitic rocks, which lie upon an undulating surface 

 of arkose. But in no instance has fossil evidence been obtained of the 

 age of the beds -immediately overlying and underlying these unconformi- 

 ties, and it should be borne in mind that unconformities in continental 

 deposits are common. 



Ehyolitic rocks, chiefly if not all tuff, were extravasated on an uneven 

 surface of arkose in the extreme upper part of the Dawson. Remains 

 of these rocks cap a number of buttes, which are prominent features of 

 the topography between Castle Rock and Palmer Lake. (Xot shown on 

 the map.) In the greater part of the area the beds above the rhyolitic 

 rocks have been removed by erosion, but four miles southeast of Castle 

 Eock the latter are sej^arated by 20 to 30 feet of arkose from the over- 

 lying Castle Eock conglomerate, in which there are large fragments of 

 the igneous material. 



Silicified wood occurs abundantly in tlie Dawson arkose. A number 

 of leaves have been collected from scNcral localities and a few fragments 

 of bones have been found. 



Bones from the Dawson arkose were obtained by the writer in only 

 one place in the southwest one-quarter of section 2, township 14 south, 

 range 65 west, 9 miles east of Colorado Springs, at an horizon estimated 

 to be about 600 feet above the base of the formation. These bones were 

 found on a hill about 100 feet above the bed of a dry creek, apparently 

 in the immediate vicinity of the rocks in which they were entombed. The 

 collection was examined by Mr. J. W. Gidley, of the U. S. National 

 Museum, who recognized dermal phxtes of a crocodile and a mammalian 

 bone. Concerning the latter, Mr. Gidley reports : 



"The one mammal bone in the collection is the distal end of a tibia, which, 

 while not generically determinable, is characteristically creodont. and indi- 

 cates a rather hiirhly advanced species of this group. The fore and aft con- 

 cavity of its articular face, together with the considerable development of a 

 median ridge, denote a specialized type of hind foot leading toward the true 

 carnivores. From our present knowledge of the creodonts snch a tj'pe could 

 not be older than Wasatch." 



The leaves listed below, determined and in part collected by Dr. F. H. 

 Knowlton, are stated by him to be "undoubtedly Denver in age." It will 

 be observed that they come from the lower 500 feet of the arkose. Other 

 smaller lots of leaves, also referred to the Denver, have been obtained 



