GEOLOGIC TIME. 



665 



POST-CAMBRIAN MECHANICAL SEDIMENTS. 



Rate of erosion over land area 

 of 600,000 square miles. 



I foot in 3,000 years, 

 1 foot in 1,000 years, 

 I foot in 200 years, - 



Time required for re- 

 moval of 3,300 feet. 



9,900,000 years 



3,300,000 years 



660,000 years 



Rate of deposition in sea of 400,000 

 square miles, for 5,000 feet of strata. 



I foot in 1,980 years, or .006 

 inch per annum. 



1 foot in 660 years, or .09 inch 

 per annum. 



1 foot in 132 years, or .18 inch 

 per annum. 



The rate of one foot in 200 years is assumed as the most 

 probable and 660,000 years as the time required for the removal 

 and deposition of the 5,000 feet of post-Cambrian mechanical 

 sediments. 



There is one factor that may need to be taken into considera- 

 tion in estimating the time duration of the deposition of the 

 mechanical sediments of the Cambrian and pre-Cambrian of the 

 northern portion of the Cordilleran sea that would materially 

 lengthen the period. Dr. George M. Dawson describes the 

 Nisconlith series, especially in the Selkirk range of British 

 Columbia, as composed of "blackish argillite-schists and phyl- 

 lites, generally calcareous, with some beds of limestone and 

 quartzite, 15,000 feet." 1 It is correlated with the Bow River 

 series, which contains, in the upper portion, the lower Cambrian 

 fauna. The presence of these calcareous beds indicates a slower 

 rate of deposition than we have estimated for the lower portion 

 of the Cambrian series over the greater part of the Cordilleran 

 sea ; but as yet the correlation with the sediments of the Cordil- 

 eran sea is not sufficiently well established to warrant our allow- 

 ing a greater time period to the Cambrian on this account. 



Estimates from Chemical Sedime?itatio7i. — We have estimated 

 that the Paleozoic sediments of the Cordilleran sea contain 

 2,007,244,800 million tons (900 million mile-rfeet) of carbonate 

 of lime, which was derived by organic or chemical agencies from 

 the sea water to which it was contributed by the land. If oceanic 

 circulation could be excluded from the problem we might pro- 

 ^ull. Geol. Soc. Amer., Vol. II. : 1891, p. 168. 



