Geologic Time. — Walcott. 365 
where the margin of the continental plateau is touched by the Guif 
Stream. Another favorable condition is the supply of carbonate of 
lime by river water directly into the ocean in the vicinity where the 
deposition of lime is going on either through organic or inorganic 
agencies. This is well illustrated by the conditions produced by the 
Gulf Stream. The oceanic currents, passing along the northeastern 
coast of South America, sweep the waters of the Amazon through the 
Caribbean sea into the gulf of Mexico, where they meet the vast vol- 
ume of water coming from the Mississippi. These are poured out 
through the narrow straits between Florida and Cuba, and carried 
northward over the sloping margin of the continental plateau. Under 
such favorable conditions the deposit must be much greater_than in areas 
where there is little circulation and the supply of calcium is limited to 
the average which is contained in sea water. If to the preceding there 
be added extensive evaporation within a partially enclosed sea, the rate 
of deposition of matter in solution will be largely increased. 
Estimate from deposition of calcium derived from Cordilleran sea 
and the outer ocean, and from the deposition of mechanical sediments. 
The area over which calcareous deposition was going on during 
Paleozoic time we have estimated at 66,000,000 square miles, which in- 
cludes the areas of the seas over the continental platforms and those of 
the surrounding oceans. As the conditions appear to have been more 
favorable for the deposition of lime in the Cordilleran and Appalachian 
seas, we will assume that it was four times that of the open ocean.* With 
a land area of 50,000,000 square miles (ante. p. 864) and a rate of 
chemical denudation of 70 tons per square mile per annum, the 
total calcium contributed to the ocean per year during Paleozoic time 
would be 3,500 million tons, or 3.78 times as much as that estimated 
per annum at the present time, which is 925,866,500 tons (ante. p. 362). 
This would have provided 50.7 tons for deposition per annum 
per square mile in the 65,000,000 square miles of ocean and seas 
and 202.8,tons for deposition per annum per square mile in the 400,000 
square miles of the Cordilleran and 600,000 square miles of similar seas. 
On this basis 81,120,000 tons (36.4 mile-feet) were contributed per an- 
num from the ocean water to the deposit in the Cordilleran sea: adding 
to this the 42,000,000 tons (18.8 mile-feet) contributed per annum by 
the denudation of the surrounding area to the Cordilleran sea, we have 
128,120,000 tons (55.2 mile-feet) as the amount available for deposit per 
annum in the Cordilleran sea. At this rate it would have required 
16,:;00,000 years to have deposited the 2,007,244,800 million tons (H00 
million mile-feet) of calcium in the Cordilleran sea; adding to this the 
1,200,000 years estimated for the deposition of the mechanical sediments, 
we have a total of 17,500,000 years as the duration of Paleozoic time. 
In reviewing the preceding estimates we must consider that through- 
*Under the reduction of 50 per cent, for the inter bedded and intermingled mechan- 
ical sediments and 25 percent, for other material than calcium depositee! from solu- 
tion, the apparent amount of calcium deposited in the Cordilleran Bea was greatly 
reduced. If thin Barne ratio of reduction is applied to other Paleozoic limestone 
areas 1 doubt if over 1,oik>,ihh> square miles will be found to contain as large an aver- 
age amount of calcium j>er square mile as the Cordilleran area. On this account 
1,000,000 square miles is the area taken for the greater rate of deposition of calcium 
during Paleozoic time. 
