330 GEOLOGIC TIME. 



This is shown l)y the inimense iibimdiuu-e of life where the margin of 

 the continental plateau is touched by tlie d lulf Stream. Another favor- 

 able condition is the supply of carbonate of lime l)y river water directly 

 into the ocean in the vicinity where the deposition of lime is going ou 

 either through "organic or inorganic agem-ies. This is well illustrated 

 by the conditions produced by the Gulf Stream. The oceanic currents, 

 l)assing alongthe 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 volume 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 continen- 

 tal plateau. Under such favorable conditions the deposit must be 

 much greater than in areas Avhere 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 inclosed sea, the rate of deposition of matter in solution 

 will be largely increased. 



Estii)i<(te from deposition of calcium derived from CordiUenoi sea and 

 the Older ocean, and from the deposition of mecltanical sediments. — The 

 area over which calcareous deposition was going ou during Paleozoic 

 time we have estin)ated at Gr>,()0(>,(K)(> square miles, which includes the 

 areas of the seas over the continental platforms and those of the sur- 

 rounding oceans. As the conditions appear to have been more favor- 

 able for the deposition of lime in the Cordilleran and Appalachian seas, 

 we will assume that it was four times that of the open oceans.* With 

 a land area of oO,()00,000 square miles and a rate of chemical denuda- 

 tion of 70 tons per Sfjuare mile per annum, the total calcium contributed 

 to the ocean per year during Paleozoic time would be 3,500,000,000 tons, 

 or 3-78 times as uuicli as that estimated per annum at the present time, 

 which is 925,866,500 tons. This would have i)rovided 50-7 tons for de- 

 position ])er 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 nule^ 

 in the 400,000 s(]uare miles of the Cordillerau and 600,000 square miles 

 of similar seas. On this basis 81,120,000 tons (36-4 mile-feet) were con- 

 tributed per annum 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 123,120,000 tons (55-2 mile-feet) as the amount available 



* Under the retlnction of 50 per ceut for the interbedded and iutermiugled niedian- 

 ical sediments and 25 per cent for other nniterial than caleinm deposited from solu- 

 tion, Iho apparent amount of calcium deposited in the Cordilleran sea was greatly 

 reduced. If this same ratio of reduction is applied to other Paleozoic limestone 

 areas I doubt if over 1,000,000 square miles will be found to contain as large an 

 average amount of calcium per square mile as the Cordilleran area. On this account 

 1,000,000 scjuare miles is the area taken for the greater rate of deposition of calcium 

 duriui-' Paleozoic time. 



