THE CRETACEOUS PERIOD. 153 



tain, because of imperfect exploration, and the presence of later beds 

 which conceal or obscure it, and because of erosion which has removed 

 it from considerable areas which it once covered. Of the Laramie 

 in the Mackenzie valley little is known. Within the general area 

 of the Laramie deposition, especially to the west, there were numer- 

 ous islands, some large and some small, which furnished a part of the 

 sediments. Neither the size nor the shape of these islands has been 

 accurately determined. 



Lithologically, the Laramie series consists primarily of sandstone 

 and subordinately of shale; but with these clastic formations there 

 is much coal. Both shale and coal are more abundant below than 

 above, while in the upper part of the series conglomerate is not rare. 

 In general, too, beds of non-marine origin increase in importance in the 

 upper part of the series. The materials of the Laramie formation 

 seem to have been derived principally from the pre-Paleozoic rocks 

 of the mountains. This, as well as the fact that the Laramie beds 

 participated in the deformation which the Paleozoic rocks have suffered, 

 fixes the date of the principal deformative movements of the Rocky 

 mountains as post-Laramie. 



The thickness of the Laramie is estimated at 1000-5000 feet, exclu- 

 sive of the transition (Mesozoic-Cenozoic) beds to be mentioned below. 

 Some parts of the series, e.g., the coal, are such as to indicate slow accu- 

 mulation. 



Various points in the structure and surface relations of the Cre- 

 taceous of Colorado are illustrated by Figs, 396 to 398. 



Fig. 39G. — Section of Cretaceous in the plains of Colorado, showing the several for- 

 mations dipping at a low angle toward the mountains and overlain in that direc- 

 tion by later Eocene formations. Kd, Dakota formation; Kc, Colorado formation; 

 Kp (Pierre) and Ktd (Trinidad), Montana series; Kl, Laramie; Epc (Poison Canyon 

 formation) and Ech (Cuchara formation) Eocene (?). Length of the section, about 

 15 miles. (Walsenburg, Colo., folio, U.. S. Geol. Surv.) 



In a considerable area in northeastern Wyoming, and in a large 

 area farther north, 1 some of the Laramie lignite has been burned. The 



1 Allen, Proc. Boston Soc. Nat. Hist., Vol. XVI, p. 246, 1874; also Bastin, Jour, 

 of Geol., Vol. XIII, p. 408. These phenomena were also noted and correctly inter- 

 preted by Lewis and Clarke. See report of their expedition. 



