im COMPARATIVE SEUDY OF THE, LOWER, CRETA- 
CEOUS, FORMATIONS AND FAUNAS, OF THE 
CNITED STATES? 
,INTRODUCTION. 
BEsIDEs the facts of wide distribution and economic impor- 
tance the Cretaceous is notable for the problems of more purely 
scientific nature than it presents. For example, near the middle 
of Cretaceous time or at the beginning of the neo-Cretaceous 
(to adopt William’s term) there was a great transgression of the 
sea upon the land—perhaps the greatest and certainly one of the 
most clearly recorded extensive one in geologic history. During 
the Trias and Jura almost all the present area of the continent 
was above sea level, as is shown by the absence of marine strata 
of those periods, excepting in limited areas of the Rocky 
Mountain and Sierra Nevada regions. The advance of the sea 
commenced with the Cretaceous, covering nearly all of Mexico 
and extending northward in the United States to southern 
Kansas, besides encroaching on the coast range region in the 
West while the lower Cretaceous sediments were forming. 
Then there was a greater and more rapid advance until at its 
maximum extent the sea covered almost the entire area between 
the Mississippi River and the Wasatch range, extending north- 
ward to the Arctic Circle. It also washed the western slope of 
the Sierra Nevada and covered the entire coastal plain of the 
Atlantic and the Gulf. The advance was not continuous nor 
constant, however. There were retrograde movements so that 
locally fresh-water and brackish-water deposits with associated 
coal beds are interstratified with the marine formations. Before 
the close of the Cretaceous while the Laramie beds were being 
tThesis submitted in partial fulfillment of the requirements for the degree of 
Doctor of Philosophy in Columbian University, Washington, D. C., June 1897. 
579 
