THE CRETACEOUS PERIOD 



423 



tion is recorded in the series of clays and sands which lie 

 upon the chalk, and by more sands containing coal seams, 



which in turn are spread upon the 



clays. The presence of the coal seams 

 records the passing of the shore line. 

 The retreat of the sea seems to have 

 been somewhat halting, however, and 

 interrupted by occasional small ad- 

 vances, for marine strata are found 

 interbedded with the coal-bearing 

 sandstones. That the retreat was 

 exceedingly slow, and that the land 



FIG. 442. A pelecypod 

 (Inoceramus) of the Cre- 

 taceous shales character- 

 istic of the western plains. 



stood for a long time not far from 

 sea level, is suggested by the thick- 

 ness of the sediments which accumu- 

 lated and by the number of succes- 

 sive coal beds in the upper part of 

 the system. Each distinct series of 

 beds thus made has a name of its 

 own, the uppermost or coal-bearing 

 series being called the Laramie for- 

 mation. There may be as much coal 

 in the Laramie as in the Pennsylva- 

 nian system in eastern United States, 

 but on the 

 average it is 



of poorer quality, and but little of it 

 is anthracite. 



Local deposits on the Pacific coast. 

 - In California and northward at 

 various points as far as Alaska, Cre- 

 taceous sandstones and shales have 

 been recognized. They are usually 

 separated from the Comanchean strata 

 by an unconformity, because much of ^ ea 4 ^ cn ] n ' 



the coastal region was land for a with the spines removed. 



FIG. 443. A Cretaceous 

 gastropod with curiously 

 formed shell and beaded 

 ornamentation. 



