268 T. W. STANTON DAKOTA SANDSTONE PROBLEMS 



Buttes, Colorado, on the Cimmarron at Garrett, Oklahoma, and at many 

 localities in northeastern New Mexico, while farther inland, beyond the 

 range of Gryphgea, we found other typical Washita fossils on the Purga- 

 toire River south of La Junta, Colorado. On more slender paleontologic 

 evidence I ventured to identify the same horizon at Canon City. In the 

 Apishapa quadrangle Stose 18 defined the Purgatoire formation to include 

 the marine shale and the underlying sandstone and conglomerate. The 

 Purgatoire has since been recognized on stratigraphic and lithologic 

 grounds, without definite paleontologic evidence, in the Colorado Springs 

 and Castle Rock folios. 



In 1906, with the enthusiasm aroused by the previous season's field- 

 work, I examined many exposures of the Dakota and associated rocks in 

 Colorado, from the Cache la Poudre, above Fort Collins, to Canon City, 

 in the hope and expectation that a distinctive Comanche fauna would be 

 found, but that hope was not realized. The oysters, Inocerami, etcetera, 

 already mentioned, were found at many localities in northern Colorado, 

 but on account of insufficient evidence I was not then and have not since 

 been able to convince myself that they were contemporaries of the 

 Washita species of Gryphsea, Protocardia, Trigonia, Inoceramus, etcetera, 

 of the southern Colorado and southern Kansas localities. Possibly they 

 were ; but, if so, why did not more of the fauna go north with them ? The 

 difference in faunas is apparently not due to difference in salinity of the 

 water, for the northern fauna is as distinctively marine as the southern 

 one. The northern fauna has been recognized as far south as Boulder 

 (about latitude 40°), while the characteristic Washita has not been seen 

 north of the Apishapa quadrangle; so that there is an interval of more 

 than 100 miles along the strike between the limits of their known occur- 

 ience. 



When sections 9 and 10 of the chart, which are still farther apart, are 

 compared, the obvious suggestion is that the two faunas occupy the same 

 position and are contemporaneous; but that implies the unproved assump- 

 tion that the overlying and underlying sandstones, respectively, are con- 

 tinuous and contemporaneous throughout the distance between the two 

 sections. A similar comparison of the Cretaceous sections of northern 

 Texas and southern Kansas just as obviously suggests that the sandstone 

 at the base of the Trinity group in Texas is of the same age as the much 

 younger Cheyenne sandstone in Kansas. It seems to me that the evidence 

 is not yet sufficient either to prove or disprove the Washita age of the 

 earliest marine Cretaceous beds of northern Colorado, but I think it more 



ls l'. S. Geol. Survey, Geologic Atlas, Apishapa folio (no. 186). 



