SECONDARY LIMESTONE. 445 



When our knowledge shall be further extended, 

 it may become possible to combine in one work 

 the proper mineral characters of rocks with the 

 description of those once living bodies which are 

 now imbedded in them ; but it is more probable 

 that it will be found necessary to separate into 

 distinct works two subjects which interfere in 

 many important points, more than they coalesce 

 in others. 



The last subject relating to the general cha- 

 racters of the secondary limestones, is colour. In 

 those rocks which contain no organic remains, 

 and even in many of those in which they exist, 

 the colour is commonly uniform throughout. In 

 these cases, scarcely any tint is found but the 

 innumerable varieties of grey ; at one extremity 

 of which may be placed pure white, as black is 

 sometimes found at the other. Ochry yellow 

 tints are also occasionally to be observed, but 

 varieties of red are far less common. 



Where organic remains are imbedded, the 

 colours of these bodies, and those of the base, 

 are sometimes different ; the former being white 

 or light grey, while the latter are dark. In other, 



