GEOLOGIC TIME. 645 



In its effect it may be compared to the Algonkian revolution 1 

 that preceded the deposition of the Paleozoic sediments. 



With the opening of new conditions the sedimentation of 

 Mesozoic time began upon the Atlantic border -and over large 

 areas of the western half of the continent with the deposit of 

 mechanical sediments — sands, silts, etc. — during Jura-Trias time. 

 They are of a character that naturally follows a period of dis- 

 turbance 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 posi- 

 tions of the beds occurred upon the Pacific and Atlantic coasts, 

 and also, to a more limited degree, throughout the Rocky moun- 

 tain region. This does not appear to have extended over the 

 plateau region or the central belt between the 97th and 105th 

 meridians. 



The Cretaceous formations have their greatest development 

 between the 97th and 1 12th meridians in Mexico and the United 

 States, in a broad belt which extends from the boundary of the 

 latter to the northwest into the British Possessions as far as the 

 6 1 st parallel. They were of a marine origin until towards the 

 close of the period when a prolonged orographic movement 

 elevated 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 estuarian 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 out- 

 lined in the previous period. 



1 The term revolution is used to describe the culmination of a long series of 

 phenomena that finally resulted in a distinctly marked epoch in the evolution of the 

 continent. The "Appalachian revolution" began far back in the Paleozoic, and 

 culminated in the later stages of the Carboniferous and the Algonkian revolution, 

 probably began far back in Algonkian time. 



