14 SURVEY OF COLOEADO AND NEW MEXICO. 



All the evidence therefore that I have had to guide me in regard to 

 these beds along the margin of the mountain ranges has been their 

 position. 



No. 2, on the Missouri Eiver, is composed of very black plastic clays, 

 with some thin layers of limestone and sandstone, and is quite well sepa- 

 rated from No. 1 below and No. 3 above. No. 3 is composed of massive 

 layers of chalky limestone, always containing Inoceramus prohlematicus 

 and Ostrea congest a. 



Along the Kansas Pacific railroad, at Hayes City and Fort Wallace, 

 No. 3 occurs in such massive layers that it is sawed into building blocks 

 with a common saw. No. 4 is a dark ashen steel-colored laminated clay, 

 wath bluish calcareous concretions filled with shells. No. 5 is a yellow- 

 ish ferruginous arenaceous clay, with the greatest abundance of mollus- 

 cous fossils. At various localities all along the margin of the mountain 

 ranges these diAdsions of the cretaceous are far less distinctly separated, 

 and vary more or less in their structure and composition, and yet in 

 tracing them carefully and continuously from the Missouri Eiver they 

 always retain enough of their typical character, so that I have never 

 been at a loss to detect their presence at once, although after leaving 

 the Missouri Eiver we do not find any well-defined lines of separation, 

 either lithologically or i^aleontologically. 



With the commencement of the tertiary was ushered in the dawn of 

 the great lake period of the West. The evidence seems to point to the 

 conclusion that from the dawn of the tertiary period, even up to the 

 commencement of the present, there was a continuous series of fresh- 

 w^ater lakes all over the continent west of the Mississippi Eiver. As- 

 suming the position that all the physical changes were slow, progressive, 

 and long-continued, and that the earlier sediments of the tertiary were 

 marine, then brackish, then purely fresh water, we have through them 

 a portion of the consecutive history of the growth of the western conti- 

 nent, step by step, up to the j)resent time. The earliest of these great 

 lakes marked the commencement of the tertiary period, and seems to 

 have covered a very large portion of the American continent west of 

 the Mississippi, from the Arctic Sea to the Isthmus of Darien. 



As I have before stated, the first sediments were marine, then came 

 brackish water, and soon purely fresh water, as is plainly indicated by 

 the organic remains. The lower beds of the great lignite basin every- 

 where contain layers, varying from a few inches to two feet in thick- 

 ness, made up almost entirely of oyster shells, with a few other species 

 of marine or estuary types. No exclusively marine forms have as yet 

 been found to my knowledge, but as we ascend in the beds all traces of 

 the salt sea disapi^ear, and a great profusion of fresh-water and land shells 

 appear, with vast quantities of the impressions of leaves of deciduous 

 trees. Numerous beds of coal, varying in thickness from a few inches 

 . to fifteen or twenty feet, characterize this deposit. 



About the middle of the tertiary i)eriod the second extensive lake 

 commenced in the West, which we have called the White Eiver tertiary 

 basin. We believe that it commenced its growth near the southeastern 

 base of the Black Hills, and gradually enlarged its borders. I am in- 

 clined to think that this lake has continued on, almost or quite uj) to the 

 commencement of the present period ; that the light colored arenaceous 

 and marly deposits in the Park of the Upper Arkansas, in the Middle 

 Park, among the mountains at the source of the Missouri Eiver, in Texas 

 and California, and Utah, are all later portions of this great lake. The 

 upper mioceue or pliocene deposits in the Wind Eiver Yalley, near Fort 

 Bridger, and on the divide between the Platte and the Arkansas Elvers, 



