864 DCEEDING8 OF THE AMERICAN ACADEMY. 



lakes. The change from the erosion of the Cretaceous floor to the <lf|K>- 

 Bilioo of the Tertiary cover " was brought about l>y Bome modification of 

 conditions which is nol yel clearly understood. Perhaps the plain- reg 

 W as dep t the west, and the Blopes thus rendered bo gentle that the 



ima could no longer carry off the detritus which came from the moun> 

 taius, and it was d< I on the way. Perhaps a barrier was lifted at 



tli.' east, so that the base level Btood higher. Whatever the cause, the 



ims . . . filled their channels bo thai their beds lay higher than the 

 neighboring country . . . and they thus came t'> flow in succession over 

 all parts of tin- plains and to distribute their deposit widely, bo that 

 tin- whole plain in tin' district here described was covered 1>\ .-ami- and 

 gravels ln»ni^rht from the canyons ami valleys of the Rocky mountains. 

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

 lamination, [n the sand are occasional pebbles and . . . bed veL 



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

 alternate with the sand in thr lower pari of the formation, and t 1 

 are probably continuations ( ,i the lake deposits observed in Kan 

 (.".7."'. 576). 



Hawortb eoes even further than Gilbert in excluding lacustrine condi- 

 tions in his discussion of the " Origin and Mode of Formation of the 

 Tertiary" in Kansas (Univ'3 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 orrespond 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 Bteepness 

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

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

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

 the water carrying tie' sediments, lesser local lakes and lagoons and 

 swamp- and mar-he- may have existed in different places and for \ ar\ ing 

 lengths of time, lint when we consider the Kansas Tertiary a- a whole 

 and vet in detail, it must he admitted that the material- themselves have 

 many indication- of river deposits and a very few of lake dep<>-ii» 



12. Fluviatile Basin Deposits. — Broad plains frequently occupy 

 ha-in-1 ike areas enclosed by mountains. Streams flow from the enclos- 

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

 through a gorge in the bordering highlands. Such plains frequently 

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



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



