1847.] The Limestones, and Lime. 69 



us, we were taught to believe that magnesia was imparted to the 

 limestones, at a period subsequent to their consolidation; and 

 hence it was believed that magnesian limestones, especially those 

 called dolomites, were altered rocks, and had undergone a process 

 which was termed dolomitization ; which consisted in the recep- 

 tion of this earth from some preexisting magnesian rock, through 

 the agency of heat. This doctrine, however, seems to be too far 

 fetched, especially when it is quite agreeable to all the facts which 

 are known of the origin of sedimentary rocks, to consider the ma- 

 terials as existing together at the time the sediment is forming, or 

 at the time the deposition is in progress. 



Many of the beds of limestone in the primary rocks are free 

 from magnesia, and we believe that observation will supply us 

 with indications respecting its presence, though probably nothing 

 short of analysis will be perfectly satisfactory. Agreeably to our 

 own observations, as it respects the presence of this earth in the 

 primary limestones, when we have found serpentine associated 

 with the beds, or disseminated in a part of the bed, we have al- 

 ways found magnesia present. On the contrary, in those which 

 are unconnected with serpentine we have not found magnesia. 



It is quite important to have some simple test for the presence 

 of this earth, which is easily applied; inasmuch as magnesia is 

 injurious when applied to lands in a caustic state, and as it ab- 

 sorbs water and carbonic acid more slowly than pure lime, it is 

 more likely to be applied too soon; and it would result in the 

 injury of a crop the first year, after which it is probable that it 

 would act beneficially, for we believe that magnesia is as import- 

 ant to some crops as lime. Magnesian limestones indeed may al- 

 ways be regarded as important substrata for soils, and we always 

 find those soils peculiarly adapted to Indian corn and the cereals. 



But we find that we have digressed, and must return to the 

 consideration of the composition of the primary limestones, those 

 which are associated with gneiss, hornblende, mica slate, and 

 granite. 



1. Limestones of JVew York — which are usually found in con- 

 nection with granite. 



Natural Bridge, Jefferson county; color white, coarsely crys- 

 talline. It contains graphite, scapolite, pyroxene and quartz. 



Analysis. 



Carbonate of lime, - - . 98.24 



Carbonate of magnesia, . . Q 



Alumina and per oxide of iron, - .88 



Insoluble matter, - - - - .88 



100. 

 A limestone mixed with serpentine, from which the calcareous 



