Ch. XXIIL] PERSISTENCY OF MINERAL CHARACTER. 
331 
pure lacustrine formations interstratified 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 lakes 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. 
