/. Croll — Mean Thickness of the Sedimentary Eochs. 101 



near the mouths of rivers and along a narrow margin extending to 

 no great distance from the land. Did the land consist of numerous 

 small islands equally distributed over the globe, the sediment carried 

 off from these islands would be spread pretty equally over the sea- 

 bottom. But the greater part of the land-surface consists of two 

 immense continents. Consequently the materials removed by de- 

 nudation are not spread over the ocean-bottom, but on a narrow 

 fringe surrounding those two continents. Were the materials spread 

 over the entire ocean -bed, a foot removed off the general surface of 

 the land would form a layer of rock only five inches thick. But in 

 the way in which the materials are at present deposited, the foot 

 removed from the land would form a layer of rock many feet in 

 thickness. The greater part of the sediment is deposited within a 

 few miles of the shore. 



The entire coast-line af the globe is about 116,500 miles. I 

 should think that the quantity of sediment deposited bej^ond, say, a 

 hundred miles from this coast-line is not very great. No dotibt 

 several of the large rivers carry sediment to a much greater distance 

 from their mouths than a hundred miles, and ocean currents may in 

 some cases carry mud and other materials also to great distances. 

 But it must be borne in mind that at many places within the 

 hundred miles of this immense coast-line little or no sediment is 

 deposited, so that the actual area over which the sediment carried off 

 the land is deposited is probably not greater than the area of this belt 

 — 116,500 miles long and 100 miles broad. This area on which the 

 sediment is deposited, on the above supposition, is therefore equal to 

 about 11,650,000 square miles. The amount of land on the globe 

 is about 57,600,000 square miles. Consequently one foot of rock, 

 denuded from the surface of the land and deposited on this belt, would 

 make a stratum of rock five feet in thickness ; but were the sediment 

 spread over the entire bed of the ocean, it would form, as has already 

 been stated, a stratum of rock of only five inches in thickness. 



Suppose that no subsidence of the land should take place for a 

 period of, say, three millions of years. During that period 500 feet 

 would be removed by denudation, on an average, off the land. 

 This would make a formation 2,500 feet thick, which some future 

 geologist might call the Post-Tertiary formation. But this, be it 

 observed, would be only the mean thickness of the formation ; its 

 maximum thickness would evidently be much greater, perhaps 

 twice, thrice, or even four times that thickness. A geologist in 

 the future measuring the actual thickness of the formation might 

 find it in some places 10,000 feet in thickness or perhaps far more. 

 But had the materials been spread over the entire ocean bed, the 

 formation would have a mean thickness of little more than 200 feet; 

 and spread over the entire surface of the globe, would form a stratum 

 of scarcely 150 feet in thickness. Therefore, in estimating the 

 mean thickness of the stratified rocks of the globe, a formation with 

 a maximum thickness of 10,000 feet may not represent more than 

 150 feet. A formation with a mean thickness of 10,000 feet repre- 

 sents only 600 feet. 



