290 Mr Kaidinger on the Parasitic Formation of Minerals, 



as the composition of the pyramidal manganese-ore is entirely 

 unknown. From the preference given to the varieties with a 

 black streak above the pyramidal species by the miners of 

 Ihlefeld, where Prof. Rose last summer found the pyramidal 

 species to occur in a particular vein in porphyry, it would ap- 

 pear that this species contains less oxygen than the product 

 of the other kind of the decomposed hydrate. The pyrami- 

 dal manganese-ore contains no water, at least not to a consi- 

 derable extent. 



VII. Changes in Minerals containing Baryta. 



A change analogous to some of those described in the genus 

 lead-baryte, is that which affects Baryto-calcite, or the hemi- 

 prismatic hal-baryte, a mineral consisting of one atom of car- 

 bonate of lime and one of carbonate of baryta. It occurs not 

 only in perfectly formed crystals, with bright surfaces, but al- 

 so in such as have lost their original brightness, and are co- 

 vered with a coating of crystals of sulphate of baryta, consti- 

 tuting the chemical composition of the prismatic hal-baryte. 

 There are varieties, also, which still show the exact hemi- 

 prismatic form of the baryto-calcite, but, when broken, do not 

 exhibit a trace of the original foliated texture, being altogether 

 composed of a granular tissue of small crystals of heavy-spar. 

 Sulphuric acid and water must have acted jointly to effect 

 this change, but the decomposition must have proceeded slow- 

 ly. The carbonic acid is expelled by the former, and the lat- 

 ter will carry away the sulphate of lime which is thus formed, 

 leaving only the sulphate of baryta. 



The pure Carbonate of 'baryta, also, which constitutes the 

 chemical substance of the species of Witherite, is found in all 

 stages of a decomposition of the same kind ; that is, from the 

 state of a carbonate, the base enters that of a sulphate. The 

 decomposition proceeds from the surface. Perfectly bright crys- 

 tals of the substance are rare, and almost entirely confined to 

 some small drusy cavities in the interior of those large globular 

 shapes occurring at Alston-moor, which are white and opaque 

 on the outside, and more translucent and yellowish within. 

 The white coating is not, however, carbonate, but it consists 

 of a number of minute crystals of sulphate, and is of variable 



