Geologic Time. — Walcott. 361 
of limestone and quartzite, 15,000 feet."* It iB 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 Cordilleran sea is not suf- 
ficiently 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 Cordilleran sea contain 2,007,244,800 million 
tons (900 million mile-feet i 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 required to 
obtain this amount of lime from the land area tributary to the Cordil- 
leran sea. It may be well to make such an estimate on the basis 
that the area of denudation tributary to the Cordilleran sea in post- 
middle Cambrian time had 600,000 square miles from which 30,- 
000,000 tons of carbonate of lime and 12,000,000 tons of sulphate 
of lime were derived per annum:+ if we assume T. Mellard Reade"s 
rate of erosion — of 50 tons of carbonate of lime and 20 ton6 of 
sulphate of lime per square mile per annum. If all of the 42,000,- 
000 tons (equal to 18.8 mile-feet) per annum were deposited within 
the limits of the Cordilleran sea, it would have taken 47,790,000 years 
for the accumulation of the carbonate of lime now estimated to have 
been deposited in the Cordilleran sea. Such a result is manifestly a 
maximum, based on the consideration of one set of phenomena. In ad- 
dition, however, the geographic conditions appear to have been fav- 
orable to the free circulation of oceanic currents through the Cordil- 
leran sea, and the temperature was favorable to extensive evaporation 
and to the development of organic life, as shown by the occurrence of 
corals in the middle and upper portions of the Paleozoic, from the 
Mackenzie river basin on the north to southern Nevada on the south. 
These conditions would reduce the time necessary for the deposition of 
the carbonate 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 .'5.85 parts of magnesium to one of calcium and river water 
contains three parts of calcium to one of magnesium. The silica and 
alumina of the river water disappear in sea water, while the sodium is 
accumulated. It is from these considerations and the fact that lime- 
stones are so largely formed of carbonate of lime that I have taken the 
latter as a basis for estimates upon the rate of chemical sedimentation. 
'Bull. Geol. Soc. Am., vol. -i. 1891, p. 168. 
tMessrs. Murray and Ri-nanl consider thai organisms have the power of Becreting 
the carbonate of lime from the sulphate of lime contained in the B6a water by chem- 
ical reaction. For an account of the chemical action that take> place in the sea 
water see report of th'- Deep-Sea Deposits of the Challenger Kxpedition. 
