SECONDARY LIMESTONE. 443 



this property, by which they are often rendered 

 very valuable.. It must lastly be remarked on 

 this subject, that these argillaceous limestones 

 sometimes entirely lose their characters, and 

 pass into the shales by which they are accom- 

 panied. 



The presence of bitumen, commonly commu- 

 nicates a black colour to limestone, with a fetid 

 smell either on burning or on friction ; and these 

 varieties are sometimes, but not necessarily, ac- 

 companied by organic remains. Besides simple 

 bitumen, it w 7 ould however appear that some 

 other inflammable matter is occasionally con- 

 tained in them ; as is proved by the smell of sul- 

 phuretted hydrogen, and by other peculiarly fetid 

 and indescribeable odours which they sometimes 

 emit on friction. 



It does not appear that the secondary lime- 

 stones are ever the repository of any simple 

 minerals capable of modifying their characters ; 

 unless pyrites be considered an exception. The 

 metallic substances found in them belong to veins, 

 and are not here subjects for consideration. 



The organic remains found in these strata, 



