BY W. F. PETTEED, C.M.Z.S. 187 



noticed the presence of calcium and water in pycnite from 

 Altenberg, which is explicable upon the conversion theory." 



At the Shepherd and Murphy Mine, Middlesex, a 

 most interesting and remarkable form of what may be 

 presumed to be altered topaz occurs. In portions of the 

 mine it is apparently fairly abundant, forming in part 

 the cassiterite-wolframite-bearing gangue of the lode. A 

 still more advanced alteration occurs in quantity. This 

 extreme change has reached the final homogeneity, and 

 will be referred to later. The substance is commonly 

 massive and granular, but crystals are not rarely met 

 with. They usually exhibit interlocked growths, and are 

 thus rough and irregular in development. The common 

 habit of the crystals is prismatic, short, and irregularly 

 striated, much distorted and compressed ; the system is 

 apparently orthorhombic. The mineral has a slightly 

 lamellar structure, the cleavage is fairly perfect, and the 

 surfaces of cleavage faces are inclined to be pearly. Frac- 

 ture crystalline, uneven, though with a glimmering lustre. 

 Colour greenish-gray to almost olive-green, and distinctly 

 translucent at the edges. 



Crystals and irregular patches of cassiterite, and more 

 rarely wolframite, are enclosed or attached with dis- 

 seminated fluorite of various tints^ but this mineral is 

 usually pale-purple with an almost colourless nucleus, 

 and may be pseudomorphous after beryl, judging from 

 the crystallographic outlines. The least unaltered por- 

 tion is not uncommonly enclosed and surrounded by a 

 substance which is the more advanced alteration product. 

 The hardness varies from 3*5 to 4 ; specific gravity = 3*4 

 3*5. Optically, refraction and double refraction strong. 

 In thin micro sections it appears to be perfectly homo- 

 geneous. When tested in closed tube it gives off water 

 with an acid reaction upon the insertion of a strip of 

 litmus paper. When strongly heated it shows sporadic 

 phosphorescence in small patches, which may be caused by 

 disseminated particles of fluorite. On strongly heating it 

 becomes dull and somewhat paler in colour. In forceps 

 it is infusible. In powder on platinum wire it glows and 

 gives the flame of Ca. On coal in powder it does not fuse 

 or intumesce. With Nao CO 3 there is no perceptible 

 reaction. With borax bead a very slight trace of Fe is 

 observable. With nitrate of cobalt, gives fairly distinct 

 reaction for aluminium. In matrass the powdered min- 

 eral gives a distinct reaction for H2 O, showing the sub- 

 stance is hydrated, leading to the supposition that it may 



