444 SECONDARY LIMESTONE. 



are of great variety, and they form a set of most 

 important geological distinctions. Their value, 

 however, in identifying distant strata, is as yet 

 unsettled, and will probably prove far more 

 limited than was once supposed. The study of 

 these substances is of so important a nature, that 

 it would be injurious to give, in this work, that 

 which must, from its nature, be necessarily su- 

 perficial and limited. Nor is indeed our know- 

 ledge of them as yet so far advanced as to admit 

 of any unexceptionable arrangement of them, 

 even by those to whom this branch of geology 

 has formed a peculiar object of attention. That 

 department of the science is as yet nearly in its 

 infancy ; and it must even remain for a future 

 time, to decide in what manner a subject so inti- 

 mately connected with the study of zoology and 

 anatomy, is to be treated of by geological writers. 

 To attempt to combine that which is imperfectly 

 known, with those departments of which the 

 knowledge is so much further advanced, could 

 serve no useful purpose ; and would, by its 

 unavoidable imperfection, throw an air of insuf- 

 ficiency over the whole of this arrangement. 



