214 



ISLAND LIFE. 



[part I. 



How to estimate the average rate of Deposition of the Sedimentary 

 Rocks. — 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. OroU has attempted 

 to make such an estimate, but he has taken for his basis the 

 mean thickness of the rocks, which we have no means whatever 

 of arriving at, and which he guesses, allowing for denudation, to 

 be equal to the maximum thickness as measured by geologists. 

 The land-area of the globe is, according to Dr. Croll, 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 mea- 

 surement on a globe shows that 100,000 miles v/ill be nearer 

 the mark, and this has the advantage of being an easily remem- 

 bered 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 comparatively 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 uni- 

 form thickness in a direction at right angles to the coast, we 

 should probably include all areas where deposits of the maxi- 

 mum thickness are forming at the present time, along with a 

 large but unknown proportion of surface where the deposits 

 were far below the maximum thickness. This follows, if we 

 consider that deposit must go on very unequally along different 

 parts of a coast, owing to the distance from each other of the 

 mouths of great rivers and the limitations of ocean currents ; 

 and because, compared with the areas over which a thick 

 deposit is forming annually, those where there is little or none 

 are probably at least twice as extensive. If, therefore, we take 

 a width of thirty miles along the whole coast-line of the globe as 

 representing the area over which deposits are forming, corre- 

 sponding to the maximum thickness as measured by geologists, 



