326 GEOLOGIC TIME. 



series over tlie greater part of the Gordillerau sea; but as yet the corre- 

 lation with the sediments of the Oordilleran sea is not sutticiently 

 well established to warrant our allowing a greater time period to the 

 Cambrian on this account. 



Estimates from chemical sedimentation.— We have estimated that the 

 Paleozoic sediments of the Gordillerau sea contain 2,007,244,800,000,000 

 [2 thousand trillion] tons (900,000,000 mile-feet) 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 proceed directly to estimate the 

 time re(pnred to obtain this amount of lime from the land area tributary 

 to the Gordillerau sea. It may be well to make such an estimate on the 

 basis that the area of denudation tributary to the Gordillerau sea in post- 

 middle Gambrian time had 000,000 S(iuare miles, from which 30,000,000 

 tons of carbonate of lime and 12,000,000 tons of sulphate of lime were 

 derived i)er annum* if we assume T. Mellard Ileade's rate of erosion — 

 of oO tons of carbonate of lime and 20 tons of sulphate of lime per 

 square mile per annum. If all of the 42,000,000 tons (eipud to 18-8 mile- 

 feet) per annum were deposited within the limits of the Gordillerau sea, 

 it would have taken 47,790,000 years for the accumulation of the carbo- 

 nate of lime now estimated to have been deposited in the Gardilleran sea. 

 Such a result is manifestly a maxinuim, based on the consideration of 

 one set of phenomena. In addition, however, to this supply of calcium 

 the geographic conditions appear to have been favorable to the free 

 circulation of oceanic currents through the Gordillerau vsea, and the 

 temperature was favorable to extensive evaporation and to the devel- 

 opment of organic life, as shown by the occurrence of corals in the 

 Middle and Upper portions of the Paleozoic, from the Mackenzie Kiver 

 basin on the north to vsouthern Nevada on the south. These condi- 

 tions would reduce the time necessary for the deposition of the carbon- 

 ate of lime. 



Ocean water of the present time contains in solution 151,025,000 

 tons of solid matter per cubic mile, which is divided among various salts. 

 A comparison of the matter in the sea and river water shows that the 

 sea contains 3.85 parts of magnesium to 1 of calcium, and river water 

 contains 3 parts of calcium to 1 of magnesium. The silica and alumina 

 of the river water disappear in sea water, while the sodium is accu- 

 mulated. It is from these considerations and the fact that limestones 

 are so largely formed of carbonate of lime that I have taken the latter 

 as a basis for estimates upon the rate of cheaiical sedimentation, an 

 allowance being made for the j^resence of silica, alumina, and magne- 

 sium in the limestones. 



*Messrs. Murray and Renard consider that organisms have the power of secretiug 

 the carbonate of lime from the sulphate of lime coutained in the sea -water by chem- 

 ical reaction. For an account of the chemical action that takes place in the sea 

 ■water see report of the Deep-sea Deposits of the Challenger Expedition. 



