358 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The materials of the Arapahoe and Denver formations seldom suggest 

 typical lake deposits. "The lower 50 to 200 feet [of the Arapahoe 

 formation] were conglomerates, the upper 400 to 600 feet arenaceous 

 clays" (31). " In passing eastward . . . the conglomerates are gradually 

 replaced by sandstones " (153). " The lower 400 feet of the [Denver] 

 series are composed entirely of eruptive debris ; above this point Archean 

 and sedimentary debris are found in small but increasing proportion, 

 and above 900 feet the material derived from . . . Archean rocks is 

 largely predominant " (33). The debris here referred to is elsewhere 

 described as largely conglomeratic near the mountains; on advancing 

 over the Plains, the sediments become finer, but still contain ^^lentiful 

 coarse sands and occasional pebbles, with numerous alternations between 

 fine conglomerate, grits, sandstones, and clays (180, 193, 195). 



Repeated instances are given of structures that are much more sug- 

 gestive of fluviatile than of lacustrine origin. " That the Denver beds 

 were deposited in shallow waters is shown by the frequent cross bedding 

 observable both in sandstone and conglomei'ate " (33). In the foot- 

 hills, " the sandy parts of the bed develop in places to wedge-shaped 

 masses exhibiting in their relations to each other and to the conglomerate 

 a very marked cross-bedding" (163). "The coarser-grained beds show 

 cross-bedding" (165). Describing a local clay deposit occurring as a 

 break in a conglomerate layer, it is remarked : " Probably the conglom- 

 erate succeeding it was dejjosited in turbulent waters" (177). "The 

 study of the conglomerate series made it evident that fine-grained beds 

 of local development might occur at almost any horizon" (177). On 

 the Plains at ten or twenty miles from the '• shore line," special mention 

 is made of " the irregular unconformable contact so frequently seen to 

 exist between a conglomerate or grit layer above and a clay or shale 

 below. . . . Often the unconformability is very marked. . . . The 

 changes in conditions of sedimentation which give rise to such strati- 

 graphical relations of consecutive beds were, however, common in both 

 Denver and Arapahoe epochs. Fine sediments were often disturbed 

 and locally removed at the beginning of periods of rapid deposition of 

 coarser materials' (180, 181). 



Tlie fossils, both of plants and animals, give no clear suggestion of a 

 lacustrine origin. " Plant remains and standing tree stumps . . . 

 abound at certain horizons " (33). " The only animal remains yet 

 found in the Arapahoe beds are the bones of vertebrates of new and 

 remarkable types. These occur in the conglomerate along the foot-hills 

 and in the basal sandstones and overlying clays beneath the prairies. In 



