108 EEPOET UNITED STATES GEOLOGICAL SURVEY. 



waters could extend westward of the maiu axis of the Front Eange, but 

 they found ingress at other points. In their lithological character, the 

 strata of the Triassic formation are very constant. So thoroughly com- 

 plete is this constancy that it is no exaggeration to say that the forma- 

 tion can readily be recognized at a glance. 



JURA. 



Wherever I observed this formation within my districts, it was invari- 

 ably associated with Triassic beds. The two are inseparable in South- 

 ern and Western Colorado. Were it not for the marked difference in 

 color between Trias and Jura, it would not be easy to distinguish the two 

 from each other. Neither in general configuration of the surface where 

 the Jurassic beds are exposed, nor in the general lithological character, do 

 they present anj^ markedly distinctive features. Instead of dolomitic 

 interstrata, as in the Triassic group, we here have calcareous ones. 

 These at many localities carry fossils in great numbers. 



North of Colorado the Jura is by far better developed; its appearance 

 there is more typical, as compared with older and younger formations. 

 No identification with European horizons seems possible. If the com- 

 parison may be used, it might be said that the Western Jurassic appears 

 very much as if the remnants, incomplete, of the European standard suc- 

 cession had been utilized in making it up. Within a very limited verti- 

 cal space our Western Jura contains fossils, the representatives of which 

 in Europe are many hundred feet apart. 



CRETACEOUS. 



For this formation we accept in Colorado three groups. Experience has 

 shown that over the vast area of that State they can readily be distin- 

 guished and remain constant in their relations. So far as palseontological 

 remains are concerned, Colorado has not furnished an exceptionally rich 

 yield. The fossils that do occur, however, are sufficiently characteristic 

 and varied in form to admit of definite classification of the beds con- 

 taining them. It is not to be denied that generally the horizons in ques- 

 tion have been recognized mainly by the aid of lithological features. 

 Acceptable as this may be with regard to any given formation within a 

 restricted area, it is a method liable to lead to erroneous interpretations. 

 It becomes a matter of vital importance, therefore, to determine, as 

 speedily as possible, in how far we are justified in relying upon petro- 

 graphic characteristics, taking into consideration horizontal and verti- 

 cal distribution. Although certain shales or sandstones may be found, 

 apparently identical with others well determined, they may and fre- 

 quently do contain fossils of a totally diffeient geological age. This 

 fact must be remembered and taken into consideration when the groups 

 of the Cretaceous formation are treated of. 



DaJwta Group. — Usually the members of this group rest directly upon 

 the Jurassic beds. In that case, they are invariably perfectly conform- 

 able, partake of the same stratigraphical features, but are set off, if 

 more or less steeply inclined, in the form of hogbacks. In Southern 

 Colorado, however, we find that the Triassic and Jurassic beds are fre- 

 quently wanting, and then the Dakota sandstones immediately overlie 

 the Upper Carboniferous Group. In such cases stratigraphical con- 

 ditions occur that closely resemble unconformabilities. Throughout 

 the districts examined, the character of the Dakota Group was a 

 very uniform one. Massive sandstones near its base gradually change 



