310 GEOLOGIC TIME. 



Archean continent. From the great tliickuess of mechanical sediments 

 it was evidently a period of elevated land and rapid deundation. With 

 the close of Algonkian time extensive orographic movements occurred 

 that outlined the subsequent development of the continent. The lines 

 of the Rocky mountain and Appalachian ranges were emphasized and 

 the great basins of sedimentation west of them defined. Subsequent 

 movements have elevated the old and formed new subparallel 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 calcareous deposits accu- 

 mulated in the Cordilleran and Appalachian seas. Since Algoukian 

 time the growth of the ct)ntinent has been by the deposition of sedi- 

 ments in the bordering oceans and interior seas and lakes within the 

 limits of the continental plateau ; and it is considered tluit the relative 

 position of the continental plateau and the deep sea have not materially 

 changed during that period. How much the deposits on the continen- 

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

 largely concealed beneath the waters of the ocean. During Paleozoic 

 time the two areas of greatest known accumulation were the Appa- 

 lachian and Cordilleran seas, where 30,000 feet or more of sediments 

 were deposited. In the Cordilleran sea sedimentation was practically 

 uniuterru})ted (except during a short interval iu middle Ordovician 

 time) until towards the close of Paleozoic time. In the uorthern 

 Appalachian sea it continued without any marked unconformity, from 

 early Cambrian to the close of Ordovician time, and, south of Xew 

 York, with relatively little interruption, until the close of Paleozoic time. 

 Certain minor disturbances occurred along the eastern border of the sea, 

 but they were not of sufficient extent to affect a general conclusion — 

 which is that the depression of tlie areas of deposition within the 

 continental platform continued without reversal of the subsidence dur- 

 ing Paleozoic time. Duriug Cambrian, and, it may be, late Algonkian, 

 time, the extended interior Mississippian region was practically leveled 

 by denudation, the eroded material being carried into the Cordilleran 

 and Appalachian seas and, probably, to a sea to the south. 



The sedimentation of the Mississipi^ian area in Paleozoic time between 

 the Appalachian and the Cordilleran seas was small as compared to 

 that which accumulated in the latter. In Devonian time there does not 

 appear to have been any sedimentation in the western portion of it 

 west of the ninety-fourth meridian and east of the Cordilleran sea, and 

 it was slight in the same interval in the Appalachian sea south of the 

 thirty- seventh parallel.* There is little if any evidence in the sedi- 



*The non-occurrence of Devonian sediment has not yet been fully explained. It 

 has been suggested that the sea beyond the reach of mechanical sedimentation was 

 too deep for the de])osition of calcai'eous deposits. It is more probable tliat the sea 

 was shallow and an area of non-deposition, or that its bed was raised to form a low, 

 level laud surface at a base level of erosion that was subjected to very slight degra- 

 dation. 



