9SI ECONOMICAL MINERALOGY. 



stalline structnre, and disintegrate into grains which often have the appearance of coarse sand, 

 and which indeed in some cases are more or less mixed with siUceous particles. Such are 

 the dolomitic limestones of the counties of Westchester, Putnam, Dutchess, Jefferson, Frank- 

 lin and St. Lawrence. Not unfrequently patches of this sandy or granular limestone are en- 

 tirely destitute of vegetation ; a circumstance which is, I think, to be chiefly ascribed to the 

 mechanical constitution of the materials which compose them. 



Another important conclusion to be drawn from the facts stated in regard to the western 

 soils is, that magnesia in the mild or carbonate state, when mixed with carbonate of lime, 

 exerts a beneficial rather than a deleterious influence. It was long since stated by Mr. Ten- 

 nant, that a lime which contained magnesia was injurious to vegetation ; and he also ascer- 

 tained by experiment, that seeds sown upon a soil mixed with some calcined magnesia, either 

 died or vegetated in a very imperfect manner, and that the plants were never healthy. Hence 

 a prejudice soon arose against all magnesian limestones ; and it was thought by many agri- 

 culturalists that this earth, in whatever form it was applied to the soil, must prove injurious 

 to it. 



Davy, in his Agricultural Chemistry, has placed this subject in its true light ; for while he 

 admits the correctness of Mr. Tennant's opinions, he at the same time gives reasons which 

 induce him to believe that magnesian limestone, or carbonate of magnesia, is a fertilizing 

 agent. Thus he observes that one of the most fertile districts in Cornwall, the Lizard, is one 

 in which the soil contains carbonate of magnesia. The correctness of this observation has 

 been confirmed by Prof. Giobert, who has announced that this earth is abundant in the most 

 fruitful soils in the vicinity of Castelmonte, and that there are numerous localities of a similar 

 kind in Piedmont. So also M. De Gasparin, in his memoir on soils, states that the carbonate 

 of magnesia is contained in great quantities in the soils of the valley of the Nile ; while, ac- 

 cording to the same author, those of Bas Languedoc often furnish from eight to thirty-three 

 per cent.* 



Now carbonate of magnesia is a constituent of many of the western rocks, and must have 

 entered largely into the composition of the soils which cover them. It exists in the proportion 

 of twenty-five to thirty per cent, in the limestones of Rochester, Monroe county, and in those 

 of Lockport in the county of Niagara. It is also found in the calciferous slate of Chittenango, 

 and in all the water limestones which I have examined,! whether from Madison, Onondaga or 

 Erie counties. Now the soils in the immediate vicinity of these magnesian rocks, so far from 

 being sterile, are among the most fertile in the State. 



If the above facts are to be relied on, we shall probably have the means of arriving at some 

 certain results in regard to the action of magnesia upon soils, whether it be apphed to them 

 in the natural state as a carbonate, or in that state of causticity to which this is brought by 

 the ordinary process of lime-burning. 



The following are suggested as being thus deducible : 



* Edinburgh New Philosophical Journal, XXVII. 84. 



t See the Tables showing the composition of several of our common and hydraulic limestones, pages 74 and 82. 



