312 GEOLOGIC TIME. 



GEOGRAPHIC CONDITIONS ACCOMPANYINCf THE DEPOSITION OF PALE- 

 OZOIC SEDIMENTS IN THE CORDILLERAN SEA. 



The assumed area of the Cordilleran or Paleo-Rocky Mountain sea 

 includes over 400,000 square miles between the thirty-fifth and fifty- 

 fifth parallels. To the eastward, during lower and middle Cambrian 

 time, a land area is thought to have extended from east of the one 

 hundred and eleventh meridian across tlie continent to the Paleo- 

 Appalachian sea. This land was depressed toward the close of middle 

 Cambrian time, and the Mississippian sea expanded over the wide 

 plateau-like interior region, from the Gulf of Mexico on the south to 

 the Lake Superior region on the north; westward it penetrated among 

 the mountain ridges between the one hundred and fifth and one hun- 

 dred and eleventh meridians, laying down the upper Cambrian deposits 

 that are now found in New Mexico, Arizona, eastern Utah, the western 

 half of Colorado, Wyoming, Idaho, and Montana, and still further 

 north into Alberta and British Columbia. During Ordovician, Silu- 

 rian, Devonian, and Carboniferous time this entire Mississippian 

 region, except x)ortions in Devonian time, appears to have been cov- 

 ered by a relatively shallow sea that was co-extensive with the Appa- 

 lachian sea and that communicated freely with the Cordilleran sea.. 

 During this same age, however, the Eocky Mountain area of New 

 Mexico, Colorado, Utah, Wyoming, and Montana formed a more or 

 less well-defined boundary of ridges and islands between the Cordilleran 

 and the interior sea up to the forty-ninth parallel. To the north 

 of the latter the conditions appear to have been the same as on the 

 eastern side of the continent, where the Appalachian sea communi- 

 cated freely with the Mississipi^ian sea. From the data that we now 

 have I think that the Palezoic (Mississippian) sea extended at times 

 over nearly all of the area subseipiently covered by the Cretaceous 

 and the later formations between the G-ulf of Mexico and the Arctic 

 ocean. This belt is bounded almost continuously on the east and 

 west by Paleozoic rocks that extend from the Arctic ocean to Mexico, 

 and whether of Cambrian, Ordovician, Silurian, or Devonian age they 

 carry essentially the same fauna throughout their extent. In the 

 outcrops of lower strata that rise up through this Cretaceous area the 

 Cambrian, Ordovician, and Carboniferous rocks are found encircling 

 the pre-Paleozoic rocks. Instances in which the Archaean rocks have 

 been met with immeduitely beneath the Cretaceous in borings in 

 Dakota and Minnesota are along the eastern border of the area, next 

 to the Archa-an rocks, where it is probable that the Cretaceous over- 

 laps the Paleozoic to the Archa'an. 



The western side of the Cordilleran sea seems to have been bounded 

 by a land area that separated it from the I'aleozoic sea, which extended 

 through central California and the Pacific border of British Columbia 

 and Yancouvers Island. From the position of the Carboniferous 



