GEOLOGIC TIME. 311 



ments of Paleozoic time to show that they were deposited in the deep, 

 open ocean; on the contrary, they were largely accumulated in partially 

 inclosed seas or mediterraneans and on the borders of the continental 

 plateau. The former is particularly true of the sedimentation of the 

 Cordilleran and Apjjalachian seas and the broad Mississippian sea. 



The close of the prolonged period of Paleozoic sedimentation was 

 brought about by what Dana has termed the "Appalachian revolution." 

 The topography of the continent was more or less changed, and the 

 conditions of sedimentation that followed were unlike those that pre- 

 ceded. This revolution raised above the sea level a considerable j)or- 

 tion of the Cordilleran and the Appalachian sea beds and also of the 

 Mississij)pian sea, east of the ninety- sixth meridian and north of the 

 thirty-fcuirth parallel. In its effect it may be compared to the Algon- 

 kian revolution* that preceded the deposition of the Paleozoic sedi- 

 ments. 



With the opening of new conditions the sedimentation of the Meso 

 zoic time began upon the Atlantic border and over large areas of the 

 western half of tlie continent with the deposit of mechanical sedi- 

 ments — sands, silts, etc. — during Jura-Trias time. They are of a char- 

 acter that naturally follows a period of disturbance of pre-existing 

 conditions and the formation of new basins of deposition with more or 

 less elevated adjoining land areas. At its close orographic movements 

 affecting the positions of the beds occurred upon the Pacific and 

 Atlantic coasts and also, to a more limited degree, throughout the 

 Eocky Mountain region. This does not api^ear to have extended over 

 the plateau region or the central belt between the ninety-seventh and 

 one hundred and fifth meridians. 



The Cretaceous formations have their greatest development between 

 the ninety-seventh and one hundred and twelfth meridians in Mexico 

 and the United States, in a broad belt which extends from the bound- 

 ary of the latter to the northwest into the British x)ossessions as far as 

 the sixty-first parallel. They were of a marine origin until toward 

 the close of the period when a prolonged orographic movement ele- 

 vated a large area of the continent above sea level and locally upturned 

 the Cretaceous strata in the Rocky Mountain area. The shoaling of the 

 sea was followed by the formation of great inland lakes in which fresh- 

 water deposits succeeded the marine and estuarine sediments. Over 

 the coastal regions they were of marine origin throughout. 



The Tertiary sediments deposited on the Cretaceous are marine on 

 the Atlantic, Gulf of Mexico, and Pacific coasts, and of fresh-water 

 origin in the Rocky Mountain and Great Plains areas, where they 

 were deposited in the great inland lakes outlined in the previous period. 



" The term " revolution " is used to describe the cuhninatiou of a long series of phe- 

 nomena that finally resulted in a distinctly marked epoch in the evolution of the con- 

 tinent. The ''Appalachian revolution' began far back in the Paleozoic and culmi- 

 nated in the later stages of the Carboniferous, and the Algoukian revolution probably 

 began far back in Aigonkian time. 



