ABSENCE OF STRATA. 133 



upper deposits of the lias, are, in many cases, scarcely to be 

 distinguished from each other. The same observation applies 

 to the lower beds of the lias and the upper beds of the new 

 red marls. 



It is also worthy of remark, that the lower members of any 

 given geological formation, whilst they resemble in litholo- 

 gical character the strata above them, present palaeontological 

 characters with the fauna of the rocks on which they repose. 

 Thus, among the fossils of the magnesian limestone the fish 

 bear, in the heterocercal character of the tail, a nearer 

 resemblance to the fossil remains of the carboniferous group 

 beneath, than to those of the new red sandstone above; 

 while those of the upper beds of the lower division of the 

 Silurian system present a closer relation to the types of 

 existence found in the rocks beneath, than in those which 

 occur above them. 



ABSENCE OF STRATA. The succession of the beds, though 

 never inverted, is occasionally imperfect, and certain mem- 

 bers of the series are not unfrequently deficient. This may 

 have been produced by one of two causes ; either by the strata 

 having been removed, subsequently to their deposition, or 

 by their never having been deposited in these spots. The 

 former agency admits of demonstration; the latter may 

 require explanation. The disturbances which the crust of 

 the earth has undergone have been so varied, and extensive, 

 that it is supposed that no portion now occupies its pristine 

 position ; and in many districts it is concluded that strata 

 nave undergone oscillation, similar to that of a board balanced 

 on a fulcrum. In the instance, therefore, of a missing deposit, 

 we have only to suppose a movement of this kind, and to 

 imagine that a portion of a given district has undergone 

 submergence, while an adjacent part has been elevated ; we 

 shall immediately perceive that the former has never received 

 those marine deposits which have been accumulated on the 

 latter. By this supposition we are enabled to account for 

 the greatest conceivable diiference in the mineralogical and 

 zoological characters of strata which lie contiguous to each 

 other. 



DRIFT is usually classified into two kinds : that which has 

 originated from local sources, and contains pebbles, boulders, 

 &c., derived from the formation in which it occurs ; and that 



