22 TF. Gross — Post-Laramie Deposits of Colorado. 



period between the marine Cretaceous and the Wasatch 

 Eocene, Mr. Hills gives much important original information 

 concerning the Laramie and various post- Laramie formations. 



The first one of these later deposits to be mentioned occurs 

 in western Colorado on the western slope of the Elk Moun- 

 tains. Its southern limit, as now known, is about Irwin, a few 

 miles to the westward of Crested Butte in Gunnison County. 

 It rests upon the normal Laramie carrying anthracite coal beds 

 on the northern slopes of the Anthracite range, occupies a 

 considerable area at the head of Anthracite creek, and thence 

 extends northward for 80 miles to Grand river. The develop- 

 ment on Anthracite creek comes within a district which has 

 been examined in detail by the Colorado division of the L T . S. 

 Geological Survey. In the course of this work the provisional 

 name of the "Ruby beds" was assigned to these strata on 

 account of their prominent development in Ruby Peak, near 

 Irwin, where their thickness exceeds 2,000 feet, and this name 

 is used by Mr. Hills in his address in referring to them. 



In the Irwin region the Laramie strata have a thickness of 

 about 1500 feet. They are succeeded by 2000 feet of con- 

 glomerates, sands and shales, composed almost entirely of the 

 debris of volcanic rocks of andesitic character, here much 

 hardened and metamorphosed by later eruptive rocks which 

 pierce them in numerous dikes. The basal member of the 

 series is a conglomerate of small pebbles, which is usually 

 uncomformable on a small scale with the Laramie, and also 

 exhibits a great variability in constitution. In some places it 

 consists entirely of andesitic pebbles, in others, of a mixture of' 

 such material with white, black or reddish chert pebbles, some 

 of which show cavities representing crinoid stems. Again, 

 the lower part of the conglomerate may be free from andesitic 

 pebbles. 



To the northward of the Irwin region Mr. Hills has traced 

 out the Ruby, beds, with a decreasing thickness reaching a 

 minimum of 300 feet near the northern limit on Grand river. 

 Concerning their development at this point Mr. Hills says : 

 " South of the Great Hogback at Coal'ridge, there is an abrupt 

 change in the composition of the sediments previously re- 

 garded as Laramie. The firm gray sandstones of the coal 

 measures are there succeeded by about 200 feet of soft white 

 sandstones and yellow clays, followed by about 300 feet of 

 tufaceous strata, more or less conglomeritic and usually loosely 

 aggregated, but resting on a hard, coarse basal conglomerate 

 about 40 feet thick made up wholly of eruptive debris. The 

 tufaceous beds are in turn succeeded by 600 feet or more of 

 shales and soft brownish sandstones which may be in part of 

 Wasatch age." * 



*Op. tit, p. 390. 



