100 J. Croll — Mean Thickness of the Sedimentary Rocks. 



Professor Huxley adds, " I do not know that any one is prepared 

 to raaiutain that the stratified rocks may not have been formed on the 

 average at the rate of -g-V of an inch per annum." When the rate, how- 

 ever, is accurately determined, it is found to be, not -^-^ of an inch per 

 annum, but only tsV-o- of ^^ inch, so that the 100,000 feet of rock 

 must have taken 1,440,000,000 years in its formation — a conclusion 

 which, according to the results of modern physics, is wholly inad- 

 missible. 



Either the thickness of the sedimentary rocks has been over- 

 estimated, or the rate of their formation has been under-estimated, 

 or both. If it be maintained that a foot in 14,400 years is too slow 

 a rate of deposit, then it must be maintained that the land must have 

 been denuded at a greater rate than one foot in 6,000 years. But 

 most geologists, I presume, felt surprised when the announcement 

 was first made, that at this rate of denudation the whole existing 

 land of the globe would be brought under the ocean in 6,000,000 of 

 years. 



The error no doubt consists in over-estimating the thickness of the 

 sedimentary rocks. Assuming, for physical reasons stated on a former 

 occasion,^ that 100,000,000 years limits the age of the stratified rocks, 

 and that th-e proportion of land to that of water and the rate of 

 denudation to have been on the average the same as at present, the 

 mean thickness of sedimentaiy rocks formed in the 100,000,000 years 

 amounts to only 7,000 feet. 



But be it observed that this is the mean thickness on an area 

 equal to that of the ocean. Over the area of the globe it amounts 

 to only 5,000 feet ; and this, let it be observed also, is the total mean 

 thickness formed, without taking into account what has been removed 

 by denudation. If we want to ascertain what is actually the present 

 mean thickness, we must deduct from this 5,000 feet an amount of 

 rock equal to all the sedimentary rocks which have been denuded 

 during the 100,000,000 years; for the 5,000 feet is not the present 

 mean thickness, but the total mean thickness formed during the 

 whole of the 100,000,000 years. If we assume, what no doubt 

 most geologists would be willing to grant, that the quantity of sedi- 

 mentary rocks now remaining is not over one-half of what has been 

 actually deposited during the history of the globe, then the actual mean 

 thickness of the stratified rocks of the globe is not over 2,500 feet. 

 This startling result would almost necessitate us to suspect that the 

 rate of sub-aerial denudation is probably greater than one foot in 

 6,000 years. But be this as it may, we are apt, in estimating the 

 mean thickness of the stratified rocks of the globe, from their 

 ascertained maximum thickness, to arrive at erroneous con- 

 clusions. There are considerations which show that the mean 

 thickness of these rocks must be small in proportion to their maxi- 

 mum thickness. The stratified rocks are formed from the sediment 

 carried down by rivers and streamlets and deposited in the sea. 

 It is obvious that the greater quantity of this sediment is deposited 



1 Phil. Mag. for May, 1868, p. 371. 



