CHAP. X.] 
THE EARTH’S AGE. 
213 
formed from the destruction of the sedimentary rocks of former 
ages. From the Laurentian age down to the present day the 
stratified rocks have been undergoing constant denudation.” 
This is perfectly true, and yet the mean thickness of that 
portion of the sedimentary rocks which remains may not he 
very different from that of the entire mass, because denudation 
acts only on those rocks which are exposed on the surface of a 
country, and most largely on those that are upheaved ; while, 
except in the rare case of an extensive formation being quite 
horizontal, and wholly exposed to the sea or to the atmosphere, 
denudation can have no tendency to diminish the thickness of 
any entire deposit . 1 Unless, therefore, a formation is completely 
destroyed by denudation in every part of the world (a thing very 
improbable), we may have in existing rocks a not very inade- 
quate representation of the mean thickness of all that have been 
formed, and even of the maximum thickness of the larger 
portion. This will be the more likely because it is almost 
certain that many rocks contemporaneously formed are counted 
by geologists as distinct formations, whenever they differ in litho- 
logical character or in organic remains. But we know that 
limestones, sandstones, and shales, are always forming at the 
same time ; 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 which 
the animals lived . 2 
1 Mr. C. Lloyd Morgan has well illustrated this point by comparing the 
generally tilted-up strata denuded on their edges, to a library in which a 
fire had acted on the exposed edges of the books, destroying a great mass 
of literature but leaving a portion of each book in its place, which portion 
represents the thickness but not the size of the book. ( Geological Magazine, 
1878, p. 161.) 
2 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.) 
