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 Ramsay 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 maodmum thickness, though 

 it undoubtedly is of their extent and hulk. 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 

 Rocks," ^ Dr. James Croll has dwelt on the extent of denuda- 

 tion in diminishing the mean thickness 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. 



