220 ISLAND LIFE 



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 admirable paper on " The Mean Thick- 

 ness of the Sedimentary Rocks," ^ Dr. James Croll has 

 dwelt on the extent of denudation in diminishing the mean 

 thickness of the rocks that have been formed, remarking, 

 " Whatever the present mean thickness 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 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 denuda- 

 tion." This is perfectly true, and j^et the mean thickness 

 of that portion of the sedimentary rocks which remains 

 may not be 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 Iwonzontal, and wholly 

 exposed to the sea or to the atmosphere, denudation can 

 have no tendency to diminish the thickness of any entire 

 deposit.^ 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 inadequate representation of the mean thickness of all 

 that have been formed, and even of the maximum thick- 

 ness of the larger portion. This will be the more likely 

 because it is almost certain that many rocks contempor- 

 aneously formed are counted by geologists as distinct for- 

 mations, whenever they differ in lithological character or 

 in organic remains. But we know that limestones, sand- 

 stones, and shales, are always forming at the same time ; 



^ Geological Magazine, Vol, YIIL, March, 1871. 



- Mr. C. Lloyd IMorgan has Avell 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 Maga-Jac^ 

 1878, p. 161.) 



