PBOI EEDING8 OP THE AMERICAN ACADEMY. 



The materials of the Arapahoe and Denver formations seldom bu| 

 typical lake deposits. "The lower 50 to 200 feel [>>t the Arapahoe 

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

 clays' 3 (31). " In passing eastward . . . the conglomerates are gradually 

 replaced by sandstones" | 153). "Thelower 1 < » * * feel of the [Denver] 

 . a are composed entirely of eruptive de*bris ; above this point Archean 

 and sedimentary debris are found in Bmall bul increasing proportion, 

 and above 900 feel 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 .-till contain plentiful 

 coarse sands ami occasional pebbles, with numerous alternations between 

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



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

 gestive of fiuviatile than of lacustrine origin. "That the Denver b< 

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

 observable both in sandstone and conglomerate" (33). In the foot- 

 hills. -the Bandy parts of the bed develop in places to wedge-shaped 

 masses exhibiting in their relation.- to each other and to theconglomei 

 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 deposited in turbulent waters"' (177). "The 

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

 of local development might occur al almost any horizon" (177). < )n 

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

 is made of "the irregular unconformable contact bo frequently -ecu 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 -trati- 

 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). 



The fossils, both of plants and animals, give no clear BUggt Stion of B 

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

 abound at certain horizons" (83). "The only animal remain- 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 Bandstones and overlying clays beneath the prairies. In 



