100 Mr HAIDINGER on the Parasitic Formation 



species contains less oxygen than the product of the other kind 

 of the decomposed hydrate. The pyramidal manganese-ore con- 

 tains no water, at least not to a considerable 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-pris- 

 matic hal-baryte, a mineral consisting of one atom of carbonate 

 of lime and one of carbonate of baryta. It occurs not only in 

 perfectly formed crystals, with bright surfaces, but also in such as 

 have lost their original brightness, and are covered with a coating 

 of crystals of sulphate of baryta, constituting the chemical compo- 

 sition of the prismatic hal-baryte. There are varieties, also, which 

 still shew the exact hemi-prismatic form of the baryto-calcite, but, 

 when broken, do not exhibit a trace of the original foliated tex- 

 ture, being altogether composed of a granular tissue of small 

 crystals of heavy-spar. Sulphuric acid and water must have act- 

 ed jointly to effect this change, but the decomposition must have 

 proceeded slowly. The carbonic acid is expelled by the former, 

 and the latter 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 opake on the 

 outside, and more translucent and yellowish within. The white 

 coating is not, however, carbonate, but it consists of a number of 



