the Right Rhombic Baryto-calcite, 



S75 



the analysis of Dr. Thomson and the specific gravities of the 

 several specimens analysed. 



Dr. Thomson informs me that his mode of analysis was to 

 convert the mixed earths into nitrates and digest in absolute 

 alcohol, a method sufficiently simple yet requiring consider- 

 able precautions to secure accuracy. It is barely possible 

 that a small portion of the matrix of carbonate of lime on 

 which the mineral rests may have escaped his notice and been 

 mixed with the quantity analysed. 



In my former account of this mineral I did not advert to 

 the presence of carbonate of strontia, as I had not attempted to 

 separate it quantitively. It is interesting, however, to find as 

 the above results show, that this substance is capable of re- 

 placing, indifferently, either one or both of the isomorphous 

 carbonates of lime and baryta, and in variable quantity. In 

 the oblique rhombic (Brooke's), column 6th, it replaces 2 per 

 cent, of the carbonate of lime, while in the right rhombic it re- 

 places a portion of both, and in the specimen analysed from 

 Bromley Hill about 3 per cent, of each. 



Sul-phatO' carbonate of Baryta. — Among the interesting new 

 barytic minerals described by Dr. Thomson (Mineralogy, 

 vol. i. p.l06,) from the same locality, — Bromley Hill minenear 

 Alston, — is one affecting the form of large six-sided prisms ter- 

 minated by hexagonal pyramids, consisting of 34*3 or one atom 

 of sulphate, and 64*82 or 2*2 atoms of carbonate of baryta. 

 This mineral is said by the dealers to be found abundantly at 

 the above mine; I have not however been able to obtain a 

 specimen of the true sulphato-carbonate. In the cabinet of 

 the Natural History Society of Newcastle, are two specimens 

 ticketed sulphato-carbonate, both from Bromley Hill. One 

 of these is in large obscure prisms an inch in diameter com- 

 posed of a congeries of small ones resting on a matrix of sul- 

 phate of baryta. This specimen agrees with the description 



• After heating to redness, during which it decrepitates slightly and be- 

 comes opake, it had a specific gravity = 3-639. 



