2 8 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [cull. 106. 



tlie thickness is 400 to 500 feet, but out on the plain it decreases until 

 it is less than 300 feet. 



About the middle of the dark shales there is always a calcareous 

 zone, sometimes 40 or 50 feet thick, consisting of alternating layers of 

 shale and drab limestone, the latter full of Inoceramus labiatus. The 

 upper 150 feet of dark shale frequently contains brown or drab calcare- 

 ous concretions, some of which are fossiliferous. Near Carlile Springs, 

 on the Arkansas river, 18 miles west of Pueblo, the upper part of this 

 zone yielded the following species: 



Exogyra snborbiculata. Liopistha conccntrica. 



Ostrea lugubris. Turritclla wliitei. 



Ostrea malachiteiisis. Amaiiropsis Imlbiforniis. 



Inoceramus fragilis. Xenopliora simpsoni. 



Cardiuin pauperciilum. Pugnellus fusiformis. 



Veniella mortoui. Rostellites dalli. 



Anatina lineata. Prionotropis hyatti. 



Above the Fort Benton shales there is a bed of rather coarse massive 

 gray sandstone which at Carlile Springs is almost 15 feet thick. It is 

 here capped by 1 to 2 feet of brown calcareous sandstone containing 

 Ostrea lugubris, impressions of Prionocycl 'us wyominc/ensis, and numerous 

 shark's teeth. Above this is about 40 feet of light-colored, compact 

 limestone with Inoceramus deformis, I. labiatus, and Ostrea congesta, 

 passing up into light gray and buff calcareous shales withOstrea congesta. 



The section is essentially the same as the one in the Denver region, 

 with the addition of the sandstone at the top of the Fort Benton shales. 

 This new feature is not distributed over a very large area, nor is it 

 constant even in the region under consideration, but it is rendered im- 

 portant by the fact that at some localities it is very fossiliferous and 

 contains a varied littoral fauna, part of which had previously been 

 known only in a limited area in northern Utah where its stratigraphic 

 relations were not clear. East of the mountains this sandstone was not 

 seen farther north than Colorado Springs, where it is 10 feet thick. On 

 the Arkansas river above Pueblo it is from 15 to 20 feet thick. Along 

 the foothills in the neighborhood of Canyon city its thickness varies 

 from 4 to 10 feet. At some localities in Huerfano park, where it is very 

 fossiliferous, it is 40 feet thick, and for convenience in accurately re- 

 ferring the species to their horizon I have there called it the Pugnellus 

 sandstone on account of the abundance of the characteristic fossil, 

 Pugnellus fusiformis. Usually it is a compact, massive, or heavy 

 bedded sandstone of uniform texture throughout, but at the localities 

 where it is very fossiliferous the fossils occur in thin bands and lenticu- 

 lar masses of hard calcareous sandstone, while the larger part of the 

 rock is more friable and sometimes contains partings of shale. 



This thin bed of sandstone shows that over a considerable area there 

 was a brief cessation and probably a reversal of the downward move- 

 ment of the Fort Benton sea bottom just prior to the deposition of the 

 Niobrara limestone. Both the character of the sediments and the con- 



