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neous ingredients being oxide of iron, sometin:ies a little oxide of 

 manganese, a little vegetable or animal organic matter, and water. 

 The second kind, besides all these several constituents, contains a 

 large proportion of carbonate of magnesia, from which it derives 

 peculiar and important properties. 



Those beds of the rock which possess, as I have already de- 

 scribed, a clear decided tint, especially any shade of blue, and a 

 smooth, sharp, even fracture, and close, fine grain or texture, 

 will yield almost invariably, a pure v/hite lime, admirably adapted 

 for making common mortar; but which is destitute of the pro- 

 perty of hardening under water. Some very dark varieties 

 furnish a beautifully white and pure lime, the colouring matter 

 consisting of bituminous substances, which are entirely consumed 

 in the process of burning. Such limestones are apt to emit a dis- 

 agreeable foetid smell when broken or strongly rubbed. 



The magnesian limestones constitute a very important por- 

 tion of the formation before us. Until my brother, Professor Wil- 

 liam B. Rogers, of Virginia, first analyzed these rocks, which he 

 has done extensively and systematically as they occur in that 

 State, their existence was hardly recognised, much less their re- 

 markable abundance. My own researches made in Pennsylva- 

 nia and New Jersey, confirm the fact of the magnesian character 

 of a large portion of this great limestone formation. 



Their economical value as hydraidic cements, very recently as- 

 certained to be dependent upon the presence of the magnesia, 

 gives additional interest and importance to the developement of 

 these extensive beds. 



No certain guide can be given for recognising the magnesian 

 varieties of the limestone, as only the eye of an experienced ob- 

 server will detect those nice shades of aspect, which denote the 

 presence of both the alkaline earths. The best general criterion, 

 is a certain dulness in the appearance of the surface, even when 

 freshly broken, and the absence of that fine smooth grain distinc- 

 tive of the pure varieties of the limestone. 



The recently discovered general fact, already alluded to, that 

 the property of forming a hydratdic cement depended upon the large 

 proportion of carbonate of magnesia in the limestone, was first 

 hinted at by M. Vicat, of France, and since confirmed by an 

 extensive series of analyses and experiments, carefully conducted 



