GEOLOGIC TIME. 649 



amount of matter in solution discharged into the Atlantic basin 



per annum from each square mile of area drained into it. Of 



this 49 tons consist of carbonate of lime and 5.5 tons of sulphate 



and phosphate of lime. 1 



Mechanical Sediments. — With the geographic conditions 



described as prevailing during Paleozoic time, the source of 



mechanical sediments later than the Middle Cambrian must have 



been from the broken area on the eastern side that extended 



100 to 200 miles to the eastward and to a much greater extent 



from the land along the western side of the sea. The enormous 



deposit of from 10,000 to 20,000 feet of mechanical sediments 



in early Cambrian time is explained by the assumption of 



favorable topographic conditions of denudation following the 



Algonkian revolution and the presence of a land area over the 



interior portion of the continent, and also, in all probability, 



between the western side of the Cordilleran sea and the western 



border of the continent. During this period the conformable 



pre-fossiliferous strata of the Cambrian accumulated and about 



6,000 feet of the lower fossiliferous rocks as they occur in the 



Eureka district of central Nevada. Following the depression of 



the continent, which carried down the central area and also 



introduced the upper Cambrian (Mississippian) sea into the 



Rocky mountain area of Colorado, etc., there were deposited of 



mechanical sediments in central Nevada : 



Ordovician sands, ------ 500 feet. 



Devonian fine argillaceous muds, ... 2,000 " 



Lower Carboniferous sands, - - - - 3,000 " 



Upper Carboniferous conglomerate and sands, - - 2,000 " 



7,500 " 

 making a total of 7,500 feet of mechanical sediments, the remain- 

 ing portion of the section (15,150 feet) being limestone. 



The following table exhibits the relative thickness of 



1 Total amount removed in solution per annum by rivers, 762,587 tons per cubic 

 mile of river water. Total discharge of river water per annum into the Atlantic, 3,947 

 cubic miles. Area drained, 26,400,000 square miles. Amount of carbonate of lime 

 per annum, 326,710 tons per cubic mile of river water ; of sulphate and phosphate of 

 lime, 37.274 tons. 



