GEOLOGIC TIME. 643 



or evolution of the North American continent, as the deposition 

 of mechanical sediments depends to a considerable extent on the 

 character of the adjoining land area, and chemical sedimentation 

 is also influenced by it. 



GROWTH OF THE CONTINENT. 



The Algonkian sediments were deposited in interior and bor- 

 dering seas that filled the depressions and extended over the 

 margins of the American continent. From the great thickness 

 of mechanical sediments it was evidently a period of elevated 

 land and rapid denudation. With the close of Algonkian time 

 extensive orographic movements occurred that outlined the sub- 

 sequent development of the continent. The lines of the Rocky 

 Mountain and Appalachian ranges were determined, and the great 

 basins of sedimentation west of them defined. Subsequent move- 

 ments have elevated the old and formed new sub-parallel ranges. 

 These movements were often of long duration and also separated 

 by great intervals of time, as is shown by the long-continued 

 base levels of erosion during which the great thickness of calcar- 

 eous deposits accumulated in the Cordilleran and Appalachian 

 seas. Since Algonkian time the growth of the continent has 

 been by the deposition of sediments in the bordering oceans and 

 interior seas and lakes within the limits of the continental pla- 

 teau ; and it is considered that the relative position of the conti- 

 nental plateau and the deep sea have not materially changed 

 during that period. How much the deposits on the continental 

 border have increased its area is unknown, as at present they are 

 largely concealed beneath the waters of the ocean. During Paleo- 

 zoic time the two areas of greatest known accumulation were in 

 the Appalachian and Cordilleran seas, where 30,000 feet or 

 more of sediments were deposited. In the Cordilleran sea sed- 

 imentation was practically uninterrupted (except during a short 

 interval in middle Ordovician time) until towards the close of 

 Paleozoic time. In the northern Appalachian sea it continued 

 without any marked unconformity, from early Cambrian to 

 the close of Ordovician time, and, south of New York, with 



