366 PRIMARY LIMESTONE. 



pact and the earthy ; and the mineral composition 

 requires no notice, as it is fundamentally, a 

 simple rock. It is often, however, much modi- 

 fied in aspect by the presence of unessential 

 minerals ; and the varieties thence resulting are, 

 in many cases, so remarkable, that it has been 

 thought necessary to give them distinct places in 

 the catalogue. The more rarely imbedded mine- 

 rals, are, as usual, subjoined in a distinct list. 



The only instance in which organic remains 

 have occurred to the author in primary limestone, 

 is in Sutherland, and it is described in his work 

 on the Western Islands. As conglomerate beds, 

 or portions of conglomerated rock, are frequently 

 found associated with the simple, it has been 

 judged necessary to enumerate those .varieties 

 here ; but they will also be found mentioned in 

 the general chapter on that subject. 



It is here necessary to add, that although the 

 primary limestones may generally be distin- 

 guished from the secondary, by their often con- 

 taining imbedded minerals which are not found 

 in those, and by the frequent existence of organic 

 remains in the latter, many cases occur where t 



