212 
ISLAND LIFE. 
[part I. 
these, together with the disintegrated or dissolved materials of 
pumice and volcanic dust, which being very light are carried by 
wind or by water over the widest oceans. 
From the preceding considerations we shall be better able to 
appreciate the calculations as to the thickness of stratified 
deposits made by geologists. Professor Kamsay has calculated 
that the sedimentary rocks of Britain alone have a total maxi- 
mum thickness of 72,600 feet ; while Professor Haughton, from 
a survey of the whole world, estimates the maximum thickness 
of the known stratified rocks at 177,200 feet. Now these maxi- 
mum thicknesses of each deposit will have been produced only 
where the conditions were exceptionally favourable, either in 
deep water near the mouths of great rivers, or in inland seas, 
or in places to which the drainage of extensive countries was 
conveyed by ocean currents ; and this great thickness will neces- 
sarily be accompanied by a corresponding thinness, or complete 
absence of deposit, elsewhere. How far the series of rocks found 
in any extensive area, as Europe or North America, represents 
the whole series of deposits which have been made there we 
cannot tell ; but there is no reason to think that it is a very 
inadequate representation of their maximum thickness, though 
it undoubtedly is of their extent and bulk. When we see in how 
many distinct localities patches of the same formation occur, it . 
seems improbable that the whole of the deposits formed during 
any one period should have been destroyed, even in such an area 
as Europe, while it is still more improbable that they should 
be so destroyed over the whole world ; and if any considerable 
portion of them is left, that portion may give a fair idea of their 
average, or even of their maximum, thickness. In his admi- 
rable paper on “ The Mean Thickness of the Sedimentary 
Bocks,” 1 Dr. James Croll has dwelt on the extent of denuda- 
tion in diminishing the mean thiekness of the rocks that have 
been formed, remarking, “ Whatever the present mean thick- 
ness of all the sedimentary rocks of our globe may be, it must 
be small in comparison to the mean thickness of all the 
sedimentary rocks which have been formed. This is obvious 
from the fact that the sedimentary rocks of one age are partly 
1 Geological Magazine, Vol. VIII., March, 1871. 
