2 Fro^, Johnston on the Dmo?phism of Barylo-calcitc. 



prism, and yet it bears an analogy to both. It had the oblique 

 character of the one form and the prismatic of the other : it 

 belongs to the hemiprismatic system of Mohs. 



Since it was first examined and described by Brooke and 

 Children, the baryto-calcite has been found in considerable 

 quantity in several lead-mines in Alston Moor. More recently, 

 however, it has also been met with in other localities ; but ap- 

 parently under different circumstances, and presenting a dif- 

 ferent appearance. The lead-mine of Fallowfield near Hex- 

 ham, in Northumberland, is known to modern collectors of 

 minerals as the locality where the finest specimens of cry- 

 stallized carbonate of barytes have yet been obtained. In 

 this mine a mineral has for some time been met with, crystal- 

 lized in six-sided pyramids, pure white, often transparent, hav- 

 ing occasionally a beautiful pink tinge, and sometimes opake, 

 from an incrustation, apparently of sulphate of barytes. More 

 lately the same mineral has been met with in one of the lead- 

 mines near Alston Moor, presenting the same characters, with 

 the exception of the pink tinge, which I have not observed in 

 any of the specimens I have seen from that locality. 



These crystals scratch carbonate of barytes and the oblique 

 rhombic baryto-calcite of Brooke; have a specific gravity of 

 3*76 at 60° Fah., and exhibit the right prismatic form of ar- 

 ragonite and carbonate of barytes. The observation as to the 

 form, which in the present case is the most important pro- 

 perty, has been confirmed by the examination of Professor 

 Miller of Cambridge. 



Another variety of the same mineral, found at Fallowfield, 

 has a pale cream colour and pearly lustre, and forms some- 

 times masses, more frequently round flattish concretions of 

 the size of a pea and upwards. Viewed through a microscope, 

 these concretions present an aggregation of minute triangular 

 faces, being sides of hexagonal pyramids, similar to those 

 which in the more regularly crystallized specimens sometimes 

 attain nearly half an inch in length. I have analysed both these 

 varieties, and found them to consist of the carbonates of lime 



and barytes united atom to atom — (C-f Ca) +(C + Ba) — 

 with scarcely a trace of iron and manganese. These crystals 

 have, therefore, the same composition as the oblique rhombic 

 baryto-calcite of Mr. Brooke as determined by Mr. Children. 

 They are, however, of a different form, being right rhombic 

 prisms, and belonging to the proper prismatic system of Mohs, 

 while the oblique rhombic crystals belong to the hemipris- 

 matic system. This mineral, therefore, is dimorphous. 



There is, however, a peculiarity in the dimorphism of this 

 mineral which has not, I believe, been observed in any other 



