242 PSEUDOMORPHOUS MINERALS OF NEW YORK. 



been made at generalization. But when it is borne in mind 

 that the changes which are here refeiTed to, do not take place 

 in small and rarely found crystals only, but are sometimes ex- 

 hibited in extensive rock formations, the importance of the sub- 

 ject to the mineralogist and geologist, will at once be apparent. 

 For if the cliemical composition of a mineral may be partly or 

 wholly changed while the external form remains the same, the 

 observer who is inattentive to these facts may often be led into 

 mistakes. 



The object of this communication is to present some facts in 

 regard to the parasitic formation of mineral species which I 

 have observed in the State of New York, and to offer some 

 suggestions of a general nature. 



Changes in Minerals containing Baryta. 



Haidingcr has alluded to a change which takes place in the 

 bari/to-calcite, a mineral containing one atom of carbonate of 

 lime and one atom of carbonate of baryta, namely, its conver- 

 sion into heavy spar, while the form of the original mineral 

 remains. He thinks that " sulphuric acid and water must have 

 acted jointly to effect this change, but the decomposition must 

 have proceeded slowly. The carbonic acid is expelled by the 

 former, and the latter will caiTy away the sulphate of lime which 

 is thus formed, leaving only the sulphate of baryta." 



In the county of Schoharie there are minerals containing 

 baryta, strontia and lime, which appear to have undergone a 

 variety of changes. Thus in the town of Carlisle there occurs 

 a fibrous mineral in veins between the layers of argillaceous 

 slate, which is now a pure sulphate of baryta with a specific 

 gravity of from 4.014 to 4.030, and then as pure a carbonate 

 of lime with a specific gravity not above 2.737. And between 

 these two extremes there are mixtures of the tsvo minerals in 

 almost every variety of proportion. It is probable that these 

 have originally been the same mineral; but in what manner 

 the change has been effected, I am not able at present to de- 

 termine. 



In other parts of this county, and especially at the village of 



