1821.] Crystallized Magnesian Carbonate of Li inc. 417 



visible aggregation of minute primary rhombi, with a white 

 opaque appearance, and a pearly lustre, seated upon small dia- 

 phanous crystals of quartz. Their other characters being those 

 common to the magnesian carbonates of lime, need not be 

 specified. 



In the hands of the same dealer I also observed, although not 

 for the first time in my life, a specimen of the Carburet of iron 

 with polyhedral surfaces, and possibly a result of crystallization. 

 As this substance is, I believe, not known to mineralogists 

 as a crystallized body, I shall be particular in describing its 

 form. The specimen came from Keswick, and, of course, origi- 

 nally from the Borrowdale mine of plumbago. It was offered to 

 me as the substance which the miners call the " leader to the 

 plumbago," the carburet of iron not being in a pure mass, but in 

 somewhat of a granular form, occupying cavities in white sparry 

 carbonate of lime. In several instances, the white spar encrust- 

 ing the detached portions of plumbago exhibited an inclination 

 of plane surfaces meeting at an angle, by the common goniome- 

 ter, of 118°, the plumbago itself corresponding in the inclination 

 of its planes with the exterior coating. Some years ago, the Rev. 

 Dr. Satterthwaite, of Jesus College in this University, visiting the 

 Borrowdale mine (in answer to my inquiries after a regularity of 

 structure in this remarkable mineral) obtained for me from the 

 Director of the Works, a single specimen, exhibiting the same 

 form with the same inclination of the plane surfaces. By com- 

 paring both together, I now find great reason to believe in the 

 existence of crystallized plumbago, and the crystals, supposing 

 those to be crystals which appear upon the specimens in my pos- 

 session, may be described as oblique four-sided prisms with 

 rhombic bases ; the obtuse angle of the rhombic base measuring 

 118°. 



A catalogue rahonnt of the minerals found in the Cumberland 

 mines would form a very interesting addition to our stock of 

 mineralogical information. I shall just notice a few other sub- 

 stances on account of their extraordinary beauty. Among them 

 are varieties of arragonite hard enough to make a deep incision 

 in glass ; crystals of quartz so penetrated by chlorite as to be 

 quite opaque, and of a fine emerald green colour ; the most 

 magnificent cubic crystals of green fluor spar highly transparent, 

 containing air bubbles moveable in a liquid which is believed to 

 be water, but this has. not been ascertained; stalactites of the 

 white carbonate of zinc resembling porcelain, and called clii/iahy 

 the dealers; cubo-octahedral sulphurets of lead containing anti- 

 mony, arsenic, and silver ; highly diaphanous sulphates of 

 barytes ; the variety of pearl spar called satin spar; primitive 

 crystals of lime spar, which do not contain magnesia ; together 

 with innumerable associations of crystallized quartz, filiate, and 

 carbonate of lime, in every variety of colour and form. 

 1 have the honour to be, dear Sir, 



Yours faithfully, E. D.Clarke. 



New Siriesj vol. ii, 2 r. 



