1819.] of Green Fluor Spar. 35 



or great limestone, that the finely crystallized and transparent 

 fluor-spar is found. These cavities are invested with the sub- 

 stance of the vein, and the fluor is there found completely cover- 

 ing the sides ; they are generally of inconsiderable extent, but 

 sufficiently capacious to admit a man standing upright within 

 the cavity. Thejiuor is there sometimes accompanied by cubical 

 and dodecahedral crystals of the sulphuret of lead {galena), and 

 sometimes by a little quartz; but, I believe, by no other 

 substance. 



The vein-stuff ~of the intersecting vein is a coarse kind of purple 

 fiuor and calcareous spar ; but although the workings for this 

 vein have been carried within 50 yards of the main deposits of 

 the green fluor in the other mine, and in the same limestone (the 

 richest of all the metalliferous limestones of this country) no 

 similar substance has been observed. The only metallic sub- 

 stance ever found in these veins is the common sulphuret of lead, 

 called lead glance, or galena ; no copper has been discovered in 

 the mines of Wear da If. 



Plate XCIV is a sketch of the situation of the green f nor mine, 

 showing also the manner of its intersection by the London Lead 

 Company's Mine. 



Having thus stated the geographical position and habitat of this 

 fluor, I will now proceed to detail its mineralogical characters. 



The finer crystals- are perfectly transparent. Their colour by 

 transmitted light is an intense emerald green ; but by reflected 

 light, the colour is a. deep sapphire blue; and this remarkable 

 character causes such a play of the green and the blue light to a 

 person regarding these crystals, that at first sight he is unable to 

 say which causes the more beautiful appearance, and to which 

 of these hues their real colour ought to be referred. Some of 

 the crystals are of such magnitude that their major diameter 

 measures two inches, but it rarely exceeds one inch. I have 

 said major diameter, because many of them are parallelopipedons, 

 with or without bevelled edges ; their surface most beautifully exhi- 

 biting the lines of decrement, formed by the laminae of superposition 

 upon the primary nucleus. These parallelopipedons are univer- 

 sally accompanied with twin crystals, which sometimes exhibit 

 a solid angle, salient, above the face of the original crystal ; and 

 sometimes one nearly as large as the crystal in which it is 

 imbedded ; the superior edge of the imbedded crystal being 

 always inclined at the same angle (somewhat less than 30°) to 

 the face of the other ; and its lines intersect the plane of the 

 original crystal in lines which make when produced nearly equal 

 angles with the edges or sides containing that angle towards 

 which the prominent edge of the twin crystal declines. The 

 twin crystal always displaces the parts of' the edge which it 

 intercepts, so that they are never in the same straight line, and 

 rarely in the same plane ; and this character in the crystallization 



c2 



