358 The American Geologist. December, 1893 
ward, in Idaho. Continuing to the north 450 miles, the sections meas- 
ured by Mr. R. G. McConnell give 29,000 feet of Paleozoic strata, in- 
cluding 14,000 feet of limestones.* In a "Note on the Geological 
Structure of the Selkirk Range,'' Dr. Geo. M. Dawson describes a sec- 
tion containing upwards of 40,000 feet of mechanical sediments, which 
he refers largely to the Cambrian.! 
The Paleozoic limestones extend to the north, on the line of the east- 
ern Rocky mountains, to the Arctic ocean. In latitude 55° to 00° N. 
the Devonian limestones are over 2,500 feet in thickness, and there are, 
other still lower Paleozoic rocks that have not yet been studied in de- 
tail. The Devonian limestones extend 700 miles in the valley of the 
Mackenzie, from Great Slave lake to below Fort Good Hope.J No Car- 
boniferous limestones have been described from this region. 
Tabulating the sections south from the 55th parallel and allowing 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 107,270,400,000 cubic feet for a thickness of 
0,000 feet, which, with an average of 12.5 cubic feet to the 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 the 
area gives an average of 70.5 per cent, of carbonate of lime.$ Tak- 
ing 75 per cent, as the proportion of pure carbonate of lime (after de- 
ducting 50 per cent, to allow for arenaceous and argillaceous ma- 
terial 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 a unit of volume, that is, the volume of 1 mile square and 
1 foot thick. (1 mile-foot=.79 kilometer-meter.) 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 billion mile-feet (4,740 million kilometer- 
*Geol. and Nat. Hist. Bur., Can.; Ann. Rep., 1866, pp. 17D-30D. 
tBull. Geol.Soc. Am., vol. :.', 1891, p. 168. 
tRept. Expl. Yukon and Mackenzie Riven Basi&B, N. \V. Terr.. Geol. and Nat. Bist. 
Bur. ( 'anada. vol. 4, (1888-89) 1890, pp. 13D-18D. 
SGeol. Expl. Fortieth Par., vol. 'J: Mon. U. S. tool. Survey, vol. 20. 
