CHAP. X THE EARTH'S AGE 221 



while a great difference in organic remains may arise from 

 comparatively slight changes of geographical features, or 

 from difference in the depth or purity of the water in whicli 

 the animals lived.^ 



How to Estimate the Average Rate of Deposition of the 

 Sedimentary Hocks. — But if we take the estimate of 

 Professor Haughton (177,200 feet), which, as we have seen, 

 is probably excessive, for the maximum thickness of the 

 sedimentary rocks of the globe of all known geological 

 ages, can we arrive at any estimate of the rate at which 

 they were formed ? Dr. CroU has attempted to make such 

 an estimate, but he has taken for his basis the 7nean 

 thickness of the rocks, which we have no means whatever 

 of arriving at, and which he guesses, allowing for denuda- 

 tion, to be equal to the maximum thickness as measured 

 by geologists. The land-area of the globe is, according to 

 Dr. CroU, 57,000,000 ^ square miles, and he gives the 

 coast-line as 116,000 miles. This, however, is, for our 

 purpose, rather too much, as it allows for bays, inlets, and 

 the smaller islands. An approximate measurement on a 

 globe shows that 100,000 miles will be nearer the mark, 

 and this has the advantage of being an easily remembered 

 even number. The distance from the coast, to which 

 shore-deposits usually extend, may be reckoned at about 

 100 or 150 miles, but by far the larger portion of the 

 matter brought down from the land will be deposited com- 

 paratively close to the shore ; that is, within twenty or 

 thirty miles. If we suppose the portion deposited beyond 

 thirty miles to be added to the deposits within that 

 distance, and the whole reduced to a uniform thickness in 

 a direction at right angles to the coast, we should probably 

 include all areas where deposits of the maximum thickness 



^ Professor J. Young thinks it highly probable that — '' the Lower Green- 

 sand is contemporaneous with part of the Chalk, so were parts of the 

 Wealden ; nay, even of the Purbeck a portion must have been forming 

 while the Cretaceous sea was gradually deepening southward and west- 

 ward." Yet these deposits are always arranged successively, and their 

 several thicknesses added together to obtain the total thickness of the 

 formations of the country. (See Presidential Address, Sect. C. British 

 Association, 1876.) 



"^ Mr. (now Sir) John Murray in his more careful estimate makes it 

 about 51 J millions. 



