430 SECONDARY LIMESTONE. 



encroaching on the peculiar department of the 

 geologist, v 



The same arguments are indeed here appli- 

 cable as in the case of the sandstones. The pri- 

 mary limestones exhibit peculiarities of character, 

 much more strongly marked and more various 

 than the secondary ; yet no reason thence arose 

 for treating of them under separate heads. On 

 a due examination of those which belong to the 

 latter class, it will immediately appear that the 

 most striking distinctions consist in the nature 

 of the organic substances which they contain ; 

 and that their other important differences are of 

 a geological nature ; relating, either to their order 

 of succession and association, or to their geogra- 

 phical situations. Neither of these are within 

 the limits of this work. 



With respect to many of the limestones des- 

 cribed by authors, more particularly in foreign 

 countries, there lias been great difficulty in ascer- 

 taining their nature from the descriptions, so as 

 to give them places in the catalogue of varieties. 

 It is eveu- uncertain with regard to many, whether 

 they are primary or secondary ; and this doubt 



