Chemical Denudation in Relation to Geological Time

Abstract

THIS little book is made up of three papers: one on “Geological Time;” a second on “The Geological Significance of the Challenger Discoveries;” and the third on “Limestone as an Index of Geological Time.” The last paper was read before the Royal Society in January, 1879, and the others have been read before the Liverpool Society, of which the author is a distinguished member. Although, therefore, not new, these papers are well worth reading, for a vast amount of good solid fact is environed by curious calculations, and by hypotheses of a highly exciting nature. That is to say, exciting to the prosy realistic disposition of modern geology. This meritorious work, however, is slightly depreciated by the introduction of matter which is not strictly consistent with the results of modern research. Nevertheless, on the whole, the work may be considered very satisfactory by those who believe that doubt is the mother of progress; for all the hypotheses and conclusions in it are the product of a geological imagination of the highest and most vigorous order, and are of course open to objection. In the introduction it is stated that the author, during an attempt to estimate the amount of “solid matter conveyed annually in solution” in river-water to the sea from the surface of England and Wales, had a “new modulus” come into his mind, which might enable him to gauge the vista of the immensity of past time, or rather to arrive at “a minimum limit to the age of the earth.” The result is thus stated: “If we imagine the area of England and Wales consisting of 58,300 square miles, to form one river-basin, the delivery of water by such river would be 68,450,936,960 tons, or 18.3 inches per annum, containing a total of 8,370,630 tons of solids in solution, representing a general lowering of the surface from that cause alone of.0077 of a foot per century, or one foot in 12,978 years.” Taking the “soluble denudation” of other parts of the world into consideration, Mr. Reade considers “that about loo tons of rocky matter is dissolved by rain per English square mile per annum.” This he states contains 50 tons of carbonate of lime, and twenty of sulphate of lime, &c., and proceeds: “If, as is generally supposed, the sea contains only what is washed into it from the land, and we can estimate its numeral contents in tons, we at once get a minimum measure of the age of the earth.” As Herschel states that the ocean contains 2,494,500 billions of tons of water, and the mean of Dr. Frankland's analysis gives 48.9 tons of carbonate of lime and magnesia, and 1,017 tons of sulphate of lime and magnesia in 100,000 tons, it follows, according to the author, that it would take 25,000,000 of years to accumulate the quantity of sulphate of lime and magnesia contained in sea water, but only 480,000 years to renew the carbonate of lime and magnesia, and the discrepancy is caused by the appropriation of the calcic carbonate by mollusca for their tests. The amount of visible sediment brought down mechanically by rivers, as calculated for the whole world upon the results of Humphreys and Abbot for the Mississippi, and the estimate is given at six times the amount of the soluble matters. This produces over the whole globe an amount of denudable matter equal to 600 tons a square mile per year. Going back in all time at this rate, and allowing for coast erosion, glaciers, &c., the ten miles of sedimentary strata must have occupied 526 millions of years in accumulation. The author readily disposes of Sir William Thomson and tidal retardation, and his limits of time. His calculations are “fallacious through leaving out agencies that we know are at work, and which the calculations I have submitted bring out in greater force.”