662 THE JOURNAL OF GEOLOGY. 



includes 27,878,400 cubic feet of limestone for each foot in thick- 

 ness and 167,270,400,000 cubic feet for a thickness of 6,000 feet, 

 which, with an average of 12.5 cubic feet to ton, gives 13,381,- 

 632,000 tons of limestone and impurities per square mile. The 

 result of ten analyses of clear limestones within the central por- 

 tion of area gives an average of 76.5 per cent, of carbonate of 

 lime. 1 Taking 75 per cent, as the proportion of pure carbonate 

 of lime (after deducting 50 per cent, to allow for arenaceous and 

 argillaceous material in partings of strata, etc.), there remain 

 5,018,112,000 tons per square mile ; multiplying this by 400,000 

 the result gives the number of tons of carbonate of lime that were 

 deposited in what we know of the Cordilleran sea in Paleozoic 

 time, or 2,007,244,800,000,000 tons, or two billion million tons in 

 round numbers. 



The following mode of presentation of the above was sug- 

 gested by Mr. Willis : 



In order to proceed with a calculation of the period required to form this thickness 

 of 15,000 feet of mechanical sediment plus 6,000 feet of calcareous sediment, it is 

 necessary, 1st, to compute the cubic volumes of the sediments; 2d, to estimate the area 

 from which they were derived ; and, 3d, to divide the cubic contents of the sediments 

 by this land area. The result thus obtained represents the depth of erosion required 

 to furnish the whole deposit, from which we may estimate the time under different 

 assumptions of the rate of erosion. 



But if we express amounts in cubic feet or tons the figures pass all comprehension ; 

 therefore, to simplify the statement, it is well to use a mile-foot as the unit of volume, 

 that is, the volume of one mile square and one foot thick. (1 mile-foot=.79 Kilometre- 

 metres). This is equal to 223,000 tons, if 12^ cubic feet of limestone equal one ton. 



Thus stated mechanical sediments covering 400,000 square miles and 15,000 feet 

 thick contain 6 billion mile-feet (4,740 million Kilometre-metres) ; and calcareous 

 sediments covering the same area and 6,000 feet thick correspond to 2 billion 4 hun- 

 dred million mile-feet (1,896 million Kilometre-metres). In the calcareous sediments 

 a liberal allowance of one-half may be made for arenaceous and argillaceous matter 

 in the limestone and partings, and analyses of ten clear limestones within the central 

 part of the area give a little more than 75 per cent, of carbonate of lime. Applying 

 these reductions we get 900 million mile feet (711 million Kilometre-metres) of pure 

 carbonate of lime. 



DURATION OF PALEOZOIC TIME IN THE CORDILLERAN AREA. 



Estimates from Mechanical Sedime?itation. — The land area tribu- 

 tary to the Cordilleran sea was larger before the depression of 



1 Geol. Expl. Fortieth Par. Vol. 2"; Mon. U. S. Geol. Survey, Vol. 20. 



