162 GEOLOGY OF THE DENVER BASIN. 



Mountain, and while the slopes of North Mountain are steeper and higher 

 the most instructive exposures are all upon South Mountain, or adjacent 

 to it. 



Exposures upon the western slopes. — Were tlie rocks under discussion a little 

 better adapted to resist the disintegrating tendencies of ordinary weather- 

 ing, there would probably be continuous sections at man) points. For 

 instance, on the ridge running west from Castle Rock toward Golden, 

 the coating of debris upon the slopes is very thin and the presence of 

 Denver strata can be demonstrated at almost any spot, although the 

 character of the stratum may not be well shown. As it is, the study 

 of the footpatli leading from Golden to Castle Rock, following in the 

 main the ridge just mentioned, will give a very good idea of the essential 

 characteristics of the formation. 



The lowest stratum clearly shown is that exposed by the ditch at the 

 end of the ridge nearest town. This is perhaps To feet above the outcrop 

 in Khmer Run which has been mentioned as the possible base of the 

 formation. The rock is yellowish in color, consisting of small pebbles, 

 gravel, and sand with some clay as matrix, and crumbles easily. Some 

 pebbles reach an inch in diameter and prove to be hornblende-andesites 

 of fresh condition and typical structure. One of these contains CI. 25 per 

 cent Si< )j, and in this rock tridvmite is very abundant. With a hand lens 

 one can recognize glassy feldspar, augite, biotite, and hornblende particles, 

 and a few rounded quartz grains in the finer material. 



About 300 yards south of the footpath and 75 to 150 feet above the 

 ditch level there is a succession of conglomeritic and sandy beds exposed 

 by a small gully. The greater part of this exposure consists of more or less 

 friable sandstone or tuff composed entirely of debris of andesites. The 

 microscope shows glassy feldspar, biotite, augite, hornblende, ore, and 

 small, worn particles of corresponding andesites. The cement is partly 

 fibrous and partly isotropic, and is described in detail in another place. 

 Pebbles of typical andesite are scattered through nearly all layers, but 

 are most abundant near the top of the section here exposed, there forming 

 a normal conglomerate. Nearly all the pebbles are small and there is a 

 great variety of types represented, all belonging in the andesitic group. 



