PALEONTOLOGY OF THE BOULDER AREA 



97 



JURA-TRIAS 



The lowest sedimentary rocks in the Boulder area are commonly 

 designated Jura-Trias, though the lower part of the supposed Triassic 

 may be in fact Permian. The entire group of stratified rocks below 

 the Cretaceous seems to be devoid of organic evidence in this locality, 

 with the exception of a single specimen, apparently a cast of a rather 

 flat bone, so that their age can at present only be determined by strati- 

 graphic relations and lithologic analogies. The Jura-Trias, up-turned 

 to a high angle, rests upon the granite, which yields to erosion and 

 atmospheric influences more readily than the sandstones and con- 

 glomerates at the base of the sedimentaries, thus forming a slight valley 

 west of the Jura-Trias, while the conglomerates and sandstones con- 

 stitute the second line of "hogbacks" or ridges along the foothills. 

 The Upper Jura-Trias, being also of yielding material, forms another 

 valley east of that ridge, separating it from the first "hogback," or 

 Dakota ridge. 



CRETACEOUS 



The Cretaceous of this region is divided into three very unequal 

 portions, called respectively the Dakota, the Colorado, and the Mon- 

 tana. The Colorado is subdivided into Fort Benton and Niobrara. 

 The Benton consists chiefly of soft shales, containing thin bands of 

 limestone and capped by a sandy zone. The base of the Niobrara is 

 a hard, thick-bedded limestone overlaid by more or less calcareous 

 shales. The Montana is subdivided into Fort Pierre and Fox Hills, 

 the former being almost entirely soft shales, with some sandstone bands 

 and limestone beds, the Fox Hills being largely friable sandstones. 



DAKOTA 



This formation, composed chiefly of hard sandstone, forms the first 

 ridge of the foothills and is paleontologically barren ground in this 

 region, though fragments of undetermined leaves and wood are found. 

 Much better preserved leaves are found at Golden, of which a collection 

 recently presented by the State School of Mines is now in the Univer- 

 sity Museum. 



