46 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA. [bull.IOG. 



tions of the section in which they were expected were much covered 

 with debris, but it is quite probable that the fresh and brackish waters 

 in which they flourished did not extend as far south and west as these 

 localities. At any rate, the formation has not been found beyond them 

 in those directions. Although the Bear River formation as such seemed 

 to be absent, nevertheless its position in the Bockport section is plainly 

 indicated by the indirect circumstantial evidence of a similar succes- 

 sion of strata and of apparently identical underlying and overlying 

 beds to be about that of the covered space Xo. 3 and the upper part 

 of No. 4. 



The lower part of the shales overlying the Bear River formation on 

 Sulphur creek are in every respect similar to No. 2 of the Bockport 

 section. In the earlier paper they were referred to the Colorado forma- 

 tion, and they seem to be fa unally connected with it, but their position 

 is now seen to be considerably lower than I then suspected, though, 

 owing to the doubt concerning the number of faults in the Bockport 

 section, I am still unable to say how many hundred feet they are below 

 the base of the Coalville section. The very great thickness of strata 

 between the top of the Colorado formation at Coalville and the base of 

 the so-called Dakota conglomerates (it can hardly be less than 5,000 or 

 6,000 feet) causes me to suspect that the latter are somewhat older than 

 the Dakota sandstone as developed east of the Rocky mountains. After 

 making such allowance for differences in the rate of deposition as the 

 character of the strata will permit and assuming that sedimentation was 

 continuous in both regions, as seems to have been the case, the 1,000 or 

 1,200 feet of strata in the one region can hardly he regarded as the com- 

 plete equivalent of 5,000 or 6,000 feet in the other. 



From the foregoing descriptions of local sections it is seen that in 

 the larger part of the area the Colorado formation rests with apparent 

 con form ability on the Dakota sandstone; that in southern Utah its 

 fauna ranges down to the local base of the Upper Cretaceous and through 

 beds that have usually been correlated with the Dakota, and that in 

 southwestern Wyoming it rests on the Bear Biver formation, with which 

 it seems to be conformable. It is also shown that the Colorado forma- 

 tion is overlain by the Montana formation with a distinct marine Cre- 

 taceous fauna. With'the exception of the two or three instances noted 

 no nonconformity has been observed between these two formations. 



The accompanying faunal lists show the geographical distribution of 

 nearly all the species and their vertical range within the area here dis- 

 cussed, and they also show the reasons for correlating the different 

 sections. 



A more general consideration of the fauna as a whole in its relations 

 to other Cretaceous faunas in this country and in Eurox)e will now bo 

 given briefly. 



