GEOLOGIC TIME, 323 



Hope.* No Carboniferous limestones have been described from this 

 region. 



Tabulating the sections south from the Fifty-fifth parallel and allow- 

 ing for a great thinning out of the sediments in Idaho and Montana, 

 we obtain an approximate general average of 21,000 feet of strata, of 

 which 6,000 feet are limestone over an area estimated to include 400,000 

 square miles. Each square mile includes 27,878,400 cubic feet of 

 limestone for each foot in thickness, and 167,270,400,000 cubic feet for 

 a thickness of 6,000 feet, which, with an average of 12-5 cubic feet to 

 the ton, gives 13,381,632,000 tons of limestone and imi^urities per 

 square mile. The result of 10 analyses of clear limestones within the 

 central portion of the area gives an average of 76-5 per cent of carbon- 

 ate of lime.t Taking 75 per cent as the proportion of pure carbonate 

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

 laceous material in partings of strata, etc.), there remain 5,018,112,0t)0 

 tons per square mile; multiplying this by 400,000 tlie 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 thousand trillion tons in round numbers. 



The following mode of presentation of the above was suggested 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 i)lns 6,000 feet of 

 calcareous sediment, it is necessary, first, to compute the cubic volumes 

 of the sediments; second, to estimate the area from which tbey were 

 derived; and, third, to divide the cubic contents of the sediments by 

 this land area. The result thus obtained rej^resents the depth of ero- 

 sion required to furnish the whole deposit, from which we may esti- 

 mate 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 a unit of volume, that is, the volume of 1 mile square and 

 1 foot thick. (1 mile-foot=0.79 kilometer-meter.) This is equal to 

 223,000 tons, if 12J cubic feet of limestone equal 1 ton. 



"Thus stated, mechanical sediments covering 400,000 s(piare miles 

 and 15,000 feet thick contain 6,000,000,000 mile-feet (4,740,000,000 kilo- 

 meter-meters) ; and calcareous sediments covering the same area and 

 6,000 feet thick correspond to 2,400,000,000 mile-feet (1,896,000,000 

 kilometer-meters). 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 10 clear limestones within the 

 central part of the area give a little more than 75 i^er cent of carbonate 

 of lime. Applying these reductions we get 000.000,000 mile-feet 

 (711,000,000 kilometer-meters) of pure carbonate of lime." 



DURATION OF PALEOZOIC TIME IN THE CORDILLERAN AREA. 



Estimates jYom mechanical sedimentation. — The land area tributary to 

 the Cordilleran sea was larger before the depression of the continent, 



* Rept. Expl. Fitkon and Mackenzie rivern Basins, y. TT., Terr., (ieol. and Xat. Bisf\ 

 Sur. Canada, (1888-89) 1890, vol. iv. pp. 1.3D-18D. 



iGeol. />j>/. Fortieth Par., vol. u; Mon, U. S, GpoJ, Surse^y, vol, ,\x, 



