Ch. XXIII.] PERSISTENCY OF MINERAL CHARACTER. 



pure lacustrine formations inter stratified with rocks older than 

 the chalk. Perhaps their absence may be accounted for by the 

 adoption of the theoretical views above set forth ; for if the 

 present ocean coincides for the most part with the site of the 

 ancient continent, the places occupied by Jakes must have been 

 submerged. It should also be recollected, that the area 

 covered by lakes, at any one time,, is very insignificant in pro- 

 portion to the sea, and, therefore, we may expect that, after the 

 earth's surface has undergone considerable revolutions in its 

 physical geography, the lacustrine strata will be concealed, for 

 the most part, under superimposed marine deposits. 



Persistency of mineral character. In the same manner as it 

 is rare and difficult to find ancient lacustrine strata, so also we 

 can scarcely expect to discover newer marine groups preserving 

 the same lithological characters continuously throughout wide 

 areas. The chalk now seen stretching for thousands of miles 

 over different parts of Europe, has become visible to us by the 

 effect, not of one, but of many distinct series of movements. 

 Time has been required, and a succession of geological periods, 

 to raise it above the waves in so many regions ; and if calca- 

 reous rocks of the Eocene or Miocene periods have been formed, 

 preserving an homogeneous mineral composition throughout 

 equally extensive regions, it may require convulsions as nu- 

 merous as all those which have occurred since the origin of the 

 chalk, to bring them up within the sphere of human observa- 

 tion. Hence the rocks of more modern periods may appear of 

 partial extent, as compared to those of remoter eras, not because 

 there was any original difference of circumstances throughout 

 the globe when they were formed, but because there has not 

 been sufficient time for the development of a great series of 

 subterranean volcanic operations since their origin. 



At the same time, the reader should be warned not to place 

 implicit reliance on the alleged persistency of the same mineral 

 characters in secondary rocks *. When it was first ascertained 

 that an order of succession could be traced in the principal 



* See some remarks on this subject, vol. i. p, 90, and Second Edition, p. 102. 



