TO JUNCTION OF GRAND AND GREEN RIVERS. 23 



traverse, a1 least through a part of their courses, channels which they have excavated 

 in the "high prairie.'- 



Throughout the greater pari of the area drained by these streams the geological 

 substratum of the plateau 1 have described is the Lower Cretaceous sandstone, or, 



more properly, a group of strata, though sometimes consisting of but a single bed, 



which forms the base of the Cretaceous system; No. 1 of Meek and Hayden's Ne- 

 braska section of the Cretaceous series. This group is composed principally and some- 

 times exclusively of thick-bedded, coarse-grained, ferruginous sandstone, containing 

 as characteristic fossils a large number of species of angiospermous leaves. Around 

 the edge of the high prairie this sandstone caps or composes many isolated buttes which 

 have been severed from their connection with-the plateau by aqueous erosion. Such, 

 WC learn from Messrs. Meek and Harden, is the structure of the Smoky Hills, from 



which they obtained a large part of the impressions of leaves described by the writer, 



and such also is the structure of Pawnee Rock, which stands near the road be- 

 tween Walnut ('reek and Pawnee Fork. At Allison's ranch, on Walnut Creek, this 

 sandstone has precisely the lithological characters of that from the Smoky Hills, but 

 I was not able to delect in it any traces of fossils. At Walnut ( "reek, however, twenty- 

 live miles distant, vegetable impressions are abundant, including, apparently, some of 

 the same species obtained from Smoky Hill, Blackbird Hill, <Sic. 



The geological horizon marked by this Lower Cretaceous sandstone group is per- 

 haps the best defined of all in the entire geological series in the Southwest, and more 

 orenerallv useful as a plane of reference than any other. From the resistant character 

 ol* the materials composing this group, it has held its place over an immense extent ol 

 Country from which the softer superior strata have been removed. When the upper 

 members of the Cretaceous formation are present, as is the case in much of the coun- 

 try hereafter to be described, the Lower Cretaceous sandstones are covered" wit h more 

 than a thousand feet of limestones, or calcareous shales tilled with the remains of ma- 

 rine organisms, and evidently a deposit from the waters of the ocean, in an analysis 

 of the Cretaceous formation, to be given in a succeeding chapter of this report, I shall 

 attempt to deduce, from the composition and fossil contents of the different members 

 of this series, something ol' the history of the changing phases of the physical geog- 

 raphy ot* the central portion of the continent during their deposition. 1 may say a 

 however, in passing, thai these coarse Cretaceous sandstones are exclusively mechan- 

 ical deposits, and such as have not been transported any very great distance from their 

 place of origin; that, extending as they do from the vicinity ol' the Mississippi all the way 

 to the base of the Rocky Mountains,* they mark a period of general subsidence in all this 

 portion of the continent— a period through which the sea was gradually encroaching 



upon the land. 'The shore-line was then constantly marching inland, leaving behind it 

 proofs of the power of its littoral waves: which comminuted and sifted the barriers op- 

 posing their progress, and formed of their ruins these beds ol' stratified sands and 

 pebbles, which may be regarded as only an unbroken series of ancient sea-beaches. 

 The liner materials, washed by the shore-waves, were taken into suspension or solu- 

 tion by the sea-water, and, mingled with or composing the remains of marine animals, 

 were Spread Over the ocean bottom as the shales and limestones of which 1 have before 



