256 DESCRIPTIVE MINERALOGY. 



Cayuga and Chautauque, often contain, in the seams which run through them, minute crys- 

 tals of dark coloured pearl spar. 



HYDRAULIC LIMESTONE. 



This substance, as found in the State of New-York, is almost always a compound of 

 carbonates of lime and magnesia, together with variable proportions of clay or silicate of 

 alumina. For a full description and notices of our localities, the reader is referred to page 

 75 of this work. 



HYDROBROMATE OF MAGNESIA. 



Bromide of Magnesium, nf Chcmisls. 



This salt, which has heretofore been found only in very minute quantity in sea water and 

 in the water of certain springs, occurs in acicular prisms, very soluble in water and alcohol. 

 It is bitter, and very deliquescent. The pure bromide, when exposed to heat, or to the air, is 

 said to be resolved into magnesia and hydrobromic acid. Formula MgBr. 



