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. Croll 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 will 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, 
