52 LIFE ON THE EARTH. 



nesses of the systems of strata, but yet (as will be 

 shewn hereafter) undoubtedly to some extent repre- 

 sented by them. 



In this scale the total thickness of the Fossil- 

 iferous strata, down to the Lingula beds of Wales, 

 is assumed to be 10 miles, 52800 ft. That is not the 

 maximum thickness, which in Britain is supposed by 

 Ramsay to be 72584 ft. In his estimate the Palae- 

 ozoic beds are taken much above the average, and 

 the Csenozoic beds much below the average of 

 Europe, Morris gives the maximum numbers thus, 

 Caanozoic 2830, Mesozoic 6170, Palaeozoic 49460. 

 D'Orbigny's general statement gives 21260 metres 

 = 69750 ft., the Tertiary occupying 9842 ft., while in 

 England they are supposed to be 2240. The figures 

 on the left may be used to mark miles of depth, thick- 

 ness of deposits, or periods of unknown duration, 

 according to the purpose in view. 



The Geological Scale of Time thus constituted by 

 the succession of marine strata, is liable to the objec- 

 tion which applies to almost every scale of historical 

 time, that it is not complete in any one region, no 

 one oceanic basin having been yet discovered which 

 has received marine sediments continuously through 

 all geological periods. The remedy is the same as in 

 ordinary history the scales of different regions are 

 combined by means of common terms, which in one 



