ENDLicH.] CONCLUSION CRETACEOUS ^POST-CRETACEOUS. 109 



into thinner beds, separated from each other by bands of shale. Near 

 the top of the group thin seams of coal set in. At a number of local- 

 ities this coal resembles a semi-anthracite. This is due in part to its 

 greater age, in part to the influence of volcanic material that has trav- 

 ersed the beds. As a rule, the Dakota sandstones, if displaced from 

 their normal horizontal position, form prominent features of the land- 

 scape. Hard as the strata are, they can successfully resist disintegra- 

 tion, and present an appearance in the hogbacks at once striking and 

 characteristic. 



Colorado Group. — This middle group of the Cretaceous is thoroughly 

 characteristic in every respect. Lithologically it is definitely distin- 

 guished from all underlying formations, and its fossils are such as to 

 readily admit of identification. It is a noticeable fact that as we pro- 

 ceed southward in Colorado the vertical dimensions of the Colorado 

 Group are subject to change. We there find that the thickness of the 

 beds is by far increased, that the nature of shales is somewhat different, 

 and that altogethr^r the group is a more prominent one than farther 

 north. A striking peculiarity of these shales is the increased dip they 

 show along the edges of any mountain range or chain. It is evident 

 that the gradual rise of the mountains, requiring more surface-area than 

 before the elevation, must have had a very great effect upon the adjoin- 

 ing sedimentary groups. Incident upon this rise is the tilting of the 

 beds. The physical constitution of the shales appears to have been 

 such that they were more siHceptible to such influences than the under- 

 and over-lying strata. It may be that a process of mechanical or chem- 

 ical hydration caused the shales to expand in a vertical dimension, thus 

 producing an eftect more than commensurate with the elevation initiat- 

 ing the change of position. On the other hand it cannot be denied that 

 the general appearance of such localities is one that at first glance sug- 

 gests a depression subsequent to the main elevation. Both causes, per- 

 haps, have combined to produce the result observed. 



Near the highest strata of the Colorado Group we again find seams 

 and beds of coal. They are mostly valueless, being too small to be 

 worked. 



Fox Hills Group. — With this division the Cretaceous, formation is 

 closed. As belonging to it we count the extensive series of shales and 

 sandstones reaching from the Colorado Group to the "Hgnitic" beds. 

 Coal is found at several horizons in the Fox Hills Group within Colorado. 

 Good workable beds occur on the Animas and at other places. The group 

 is a characteristic one, retaining what may be termed a " Cretaceous 

 habitat." It is in harmony with the preceding ones lithologically and 

 palseontologically. All of these three Cretaceous groups occupy exten- 

 sive areas in Colorado, more particularly in the southern and south- 

 western portions of the State. 



POST-CRETACEOUS. 



This formation, which may be regarded as the coal-bearing series 

 proper of the Kocky Mountains, has long since been designated as the 

 Laramie Group. For reasons based upon palseontological and other 

 grounds, it is no longer referred either to the Cretaceous or Tertiary 

 formation by some of the geologists who have examined it. It is more 

 in conformity with the progress of geological science thus to regard the 

 Laramie Group as a separate formation than to assign it either to an 

 older or a younger one, with neither of which it sufficiently agrees. 

 Classification in science is but the expression of the summarized knowl- 



