672 THE JOURNAL OF GEOLOGY. 



The area over which calcareous depositions was going on 

 during Paleozoic time we have estimated at 66,000,000 square 

 miles, which includes the areas of the seas over the. continental 

 platforms and those of the surrounding oceans. As the con- 

 ditions appear to have been more favorable for the deposition of 

 lime in the Cordilleran aud Appalachian seas, we will assume 

 that it was four times that of the open ocean. 1 With a land 

 area of 50,000,000 square miles {ante p. 670) 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 for per annum at the present time, which is 925,866,- 

 500 tons {ante p. 668). 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 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 128,120,000 tons (55.2 mile-feet) as the amount avail- 

 able for deposit per annum in the Cordilleran sea. At this rate 

 it would have required 16,300,000 years to have deposited the 

 2,007.244,800 million tons (900 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, 



1 Under the reduction of 50 per cent, for the interbedded and intermingled 

 mechanical sediments and 25 per cent, for other material than calcium deposited 

 from solution, the apparent amount of calcium deposited in the Cordilleran sea was 

 greatly reduced. If this same ratio of reduction is applied to other Paleozoic lime- 

 stone 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 square miles is the area taken for the greater rate of deposition of 

 calcium during Paleozoic time. 



