Page:The Journal of geology (IA journalofgeology11893univers).pdf/236
includes 27,878,400 cubic feet of limestone for each foot in thickness of 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 portion 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 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 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 12½ 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 hundred 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 file feet (711 million Kilometre-metres) of pure carbonate of lime.
DURATIONS OF PALEOZOIC TIME IN THE CORDILLERAN AREA.
Estimates from Mechanical Sedimentation.—The land area tributary to the Cordilleran sea was larger before the depression of
- ↑ Geol. Expl. Fortieth Par. Vol. 2, Mon. U. S. Geol. Survey, Vol. 20.