191 



By calculation is obtained — 



29-93 Al. 2 Oft 

 17-46 Si. 0- 

 318 Al. 

 11-02 Fl. 

 1-62 Si. 2 



(53-21 Topaz. 

 35 '39 Quartz. 



2-42 Tin. 



03 Ca. 0. 



101-05 Total. 

 —0-34 Fl. 



pre- 

 tliis 



= 100-71 



Just as the fine white topaz from Mount Bisclioff, 

 viously examined (c.f.r. this Journal, 1884, page 647), 

 topaz also contains some lime. According to Sandberger, its 

 presence is caused by an incipient transformation from topaz 

 to prosopit. 



The topaz does not appear simply as a crust surrounding 

 the quartz crystals, but as a pseudo-morphosis from them. 

 In proof of this, the circumstance that the thickness of the 

 topaz crust is always in inverse proportion to the diameter of 

 the quartz nucleus (the entire crystals being of an even 

 diameter of from 2-3 mm.) must be considered. In some 

 crystals the topaz has entirely replaced the quartz ; in others 

 there appears a nucleus which, on breaking, is seen to be a 

 very minute point. Between crystals of this form, and others 

 only slightly surrounded by topaz, or wholly free from it, all 

 transition stages are represented. It is remarkable that 

 quartz crystals, partially transformed into topaz, have lost 

 their distinct outline and smooth surface. 



On transverse fracture of crystals containing larger quartz 

 nuclei, it is seen that the topaz Iras advanced in a very 

 irregular manner from without inwards. The crusts of topaz 

 are not of uniform thickness ; they protrude into the quartz 

 in an angular manner. This is particularly well seen in 

 thinly ground sections. The line of demarcation between 

 topaz and quartz is quite irregular. The former contains, as 

 is specially well seen under the microscope, small grains of 

 quartz. Minute crystals of topaz protrude into the clear 

 quartz in the form of needles, fibres, and bundles of fibres, or 

 even lie, apparently isolated, in among the quartz. The quartz 

 contains innumerable, irregularly distributed vacuoles, partly 

 of an irregular, partly of an angular form. (Negative 

 crystals). 



The topaz is to be distinguished from the quartz by ordi- 



