314 GEOLOGIC TIME. 



we allot 50 tons to carbonate of liuie, 20 tons to sulphate of lime, 7 to 

 silica, 4 to carbonate of magnesia, 4 to snlphate of magnesia, 1 to perox- 

 ide of iron, 8 to chloride of sodium, and to the alkaline carbonates and 

 snl]i]iates, we shall probably be as iiear the truth as present data will 

 allow us to come."* By the use of the data given by Mr. John Murray, 

 in a paper on the total annual rainfall on the land of the globe and the 

 relation of rainfall to the discharge of rivers, t I obtain 113 tons as the 

 total amouut 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 phos- 

 phate of lime. I 



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 mechan- 

 ical sediments in early Cambrian time is explained by the assumi)tion 

 of favorable topographic conditions of denudation following theAlgon- 

 kian revolution and the presence of a land area over the interior por- 

 tion of the continent, and also, in all probability, between the Avestern 

 side of the Cordilleran sea and the western border of the continent. 

 During this period the conformable pre-fossiliferous strata of the Cam- 

 brian accumnlatcd, 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 Eocky 

 Mountain area of Colorado, etc., there were deposited of mechanical 

 sediments in central Nevada : 



Feet. 



Ordovician sands 500 



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 remaining 

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



The following table exhibits the relative thickness of mechanical and 

 chemical deposits in the Cordilleran sea after the Middle Cambrian 

 subsidence : 



* Froc. Liverpool Geolog. Soc, 1877, voL iii, p. 229. 



i Scottish Geol. Mmj., 1887, vol. in, pp. 65-77. 



X Total amouut 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 

 line per annum, 326,710 tons per cubic mile of river water; of sulphate and phos- 

 phate of lime, 37;274 tons. 



