188 



see the modification, especially to an unpractised observer. 

 They often imitate the plated crystals of sulphate of barytes. 

 The present specimen is a very fine one, obtained some years 

 since. This shows that they sometimes originate from the 

 decomposition of galaena, and they are here yet coloured 

 with it. The matrix is composed of galaena mixed with 

 fluor. This very curious specimen has the first modifica- 

 tion of the quartz-like crystal, deduced from the primi- 

 tive rhomb (see the left hand outline), with the column just 

 visible: these pass into regular dodecaedrons, with very short 

 columns, or rather octo-decaedrons; and also form the 

 same figure in plates, which, if regular, show the surfaces 

 of 1 2 intersecting planes or facets : see the right hand 

 lower figure: but these are seldom quite regular, and they 

 may be so confused and indeterminate that we cannot make 

 them out: see tab. 90. The under figure is a modification 

 seen on the same specimen, formed by the primitive before 

 spoken of, having a larger deposition on some of the faces 

 than on others, which gives it a lengthened appearance. 

 Analysis by Westrumb : Oxide of lead 81*2 



Carbonic acid 



16-0 



Lime 



0-9 



Oxide of iron - 



0-3 



Loss 



1-6 



lOO'O 



