of Mineral Species. 89 



coarser texture, the explanation given of the Brazilian octahe- 

 drons. The rough form of an octahedron is produced by very 

 distinct flat crystals, united in various positions, of the rhom- 

 bohedral species, the face perpendicular to the axis of the fun- 

 damental rhombohedrons being much enlarged. Some of them 

 have their broad faces in the direction of the faces of the oc- 

 tahedron ; and in some of the octahedral groupes, this circum- 

 stance has produced a kind of raised reticulated appearance on 

 the adjoining faces of the original octahedron, which the newly 

 formed crystals intersect, and project beyond them. 



The changes which affect the brachytypous parachrose-ba- 

 ryte, or sparry iron, deserve our particular notice, as they are 

 not only highly interesting in themselves, but have been well 

 attended to at all those places where this species forms the pre- 

 dominant ore of iron. The characteristic chemical ingredient 

 of it is the carbonate of iron, Fe C 2 , in which the protoxide 

 of iron and the carbonic acid are in the ratio of 61.47 and 38.53. 

 It contains occasionally an admixture of the carbonates of lime, 

 magnesia and manganese. The colour of the original varieties 

 is usually a pale yellow, inclining to grey : the lustre and trans- 

 parency are considerable. When left exposed to the action of 

 the atmosphere, the surface soon assumes a brown tint, which 

 by degrees penetrates deeper into the substance of the crys- 

 tals. Some lustre even then remains, and cleavage is still obser- 

 vable. Specimens bounded by fissures on all sides, or broken out 

 of a solid mass, when examined in this stage of their decompo- 

 sition, often still contain a nucleus of the yellowish-grey undecom- 

 posed substance. When the decomposition has arrived at its end, 

 every trace of cleavage has disappeared, the fracture of perfectly 

 well pronounced crystalline shapes is uneven, or earthy, and the 

 colour a dark brown, which is likewise visible in its streak. The 

 substance now consists of a compact variety of the hydrate of per- 

 oxide of iron, whose chemical composition is expressed in the 



VOL. XI. PART I. M 



