364 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lakes. The change from the erosion of the Cretaceous floor to the depo- 

 silioa of the Tertiary cover " was brought about by some modification of 

 conditions which is not yet clearly understood. Perhaps the plains region 

 was depressed at the west, and the slopes thus rendered so gentle that the 

 streams could no longer carry off the detritus which came from the moun- 

 tains, and it was deposited on the way. Perhaps a barrier was lifted at 

 the east, so that the base level stood higher. Whatever the cause, the 

 streams . . . filled their channels so that their beds lay higher than the 

 neighboring country . . . and they thus came to flow in succession over 

 all parts of the plains and to distribute their deposit widely, so that 

 the whole plain in the district here described was covered by sands and 

 gravels brought from the canyons and valleys, of the Rocky mountains. 

 The chief material is coarse sand ... in irregular beds with much oblique 

 lamination. In the sand are occasional pebbles and . . . beds of gravel. 

 ... At the northeast, . . . clays, marls, and other fine-grained beds 

 alternate with the sand in the lower part of the formation, and these 

 are probably continuations of the lake deposits observed in Kansas" 

 (575, 576). 



Haworth goes even further than Gilbert in excluding lacustrine condi- 

 tions in his discussion of the " Origin and Mode of P'ormation of the 

 Tertiary" in Kansas (Univ'y Geol. Surv. Kansas, ii. 1897, 281-284). 

 After quoting extracts from Gilbert's report, this author says : " The 

 relative positions of the gravel, sand, and clay of the Tertiary over the 

 whole of Kansas . . . correspond much better to river deposits than to 

 lake deposits. The irregularity of formation succession, the limited lat- 

 eral extent of the beds of gravel, clay and sand, the frequent steepness 

 of the cross-bedding planes, all correspond to river deposits, but are not 

 characteristic of lake deposits. ... It is quite possible that during 

 Tertiary time, in which there were so many changes in the velocity of 

 the water carrving the sediments, lesser local lakes and la^oous and 

 swamps and marshes may have existed in different places and for varying 

 lengths of time. But when we consider the Kansas Tertiarv as a whole 

 and yet in detail, it must be admitted that the materials themselves have 

 many indications of river deposits and a very few of lake deposits " (283). 



12. Fluviatile Basin Deposits. — Broad plains frequently occupy 

 basin-like areas enclosed by mountains. Streams flow from the enclos- 

 ing slopes to the central depression, whence the united waters find escape 

 through a gorge in the bordering highlands. Such plains frequently 

 give the impression of having been once occupied by a lake. The plain 

 of Hungary is an admirable example of this kind, yet there is much 



