GEOLOGIC TIME. 647 



through this Cretaceous area, the Cambrian, Ordovician, and 

 Carboniferous rocks are found encircling the pre-Paleozoic rocks. 

 Instances in which the Archean rocks have been met with 

 immediately beneath the Cretaceous in borings 4n Dakota and 

 Minnesota are along the eastern border of the area, next to the 

 Archean rocks, — where it is probable that the Cretaceous over- 

 laps the Paleozoic to the Archean. 



The western side of the Cordilleran sea seems to have been 

 bounded by a land area that separated it from the Paleozoic sea, 

 which extended through central California and the Pacific border 

 of British Columbia and Vancouver's Island. From the posi- 

 tions of the Carboniferous deposits of California at the present 

 time it appears that this land varied from 100 to 150 miles in 

 width and was practically continuous along the western side of 

 the Cordilleran sea. This view is further strengthened by the 

 fact that the Carboniferous fauna of California has certain char- 

 acteristics which are not found in the Carboniferous of the Cor- 

 dilleran area. Our knowledge of conditions north of the 55th 

 parallel is limited by the want of accurate geologic data. If 

 Cambrian and Carboniferous rocks were not deposited in the 

 Mackenzie river basin and also on the eastern side of the area 

 now covered by Cretaceous strata, the inference is that during 

 Cambrian and Carboniferous time there was a land area to the 

 east and north of the northern Cordilleran sea that may have 

 been tributary to the latter. 



SOURCE OF SEDIMENTS DEPOSITED IN THE CORDILLERAN SEA. 



The sediments deposited in every sea or lake are derived 

 from land areas either by mechanical or chemical denundation. 



Mechanical denudation results from the action of the waves 

 and currents along the shore and the agency of rain, frost, snow, 

 ice, wind, heat, etc., on the land. Rain is the most important 

 factor, and the result depends mainly upon its amount and the 

 slope or the gradient of the land. The general average of 

 denudation for the surface of the land areas of the globe, now 

 usually accepted, is one foot in 3,000 years. This varies locally, 



