150 MR. J. B. SCRIVENOR ON THE [May I903, 



The secondary quartz, derived from the felspar chiefly, but 

 also in part from the biotite, forms much smaller grains than the 

 original quartz, and contains many more, though slightly smaller, 

 fluid-cavities. 



In addition to the secondary quartz, there is another mineral 

 apparently derived from the orthoclase. This is topaz, which 

 occurs in minute grains, showing characteristic refraction and double 

 refraction, but very rarely the cleavage parallel to 001 or the 

 crystal-outline. In order better to determine this mineral, some of 

 the rock was crushed, and separated in cadmium-borotungstate of 

 sp. gr. 3*2. The grains afforded by this means showed the cleavage 

 much better, and in one or two cases the prism-faces ; moreover, 

 one grain was found which gave an undoubted orthorhombic axial 

 figure with the characteristic axial angle of topaz. Prof. Zirkel 1 

 mentions a complete replacement of felspar by topaz-grains, often 

 retaining the outline of the original felspar ; but, in these slides, 

 the grains, although sometimes forming a considerable continuous 

 mass, do not give any indication of the felspar-faces. Topaz was 

 also found in the vein-quartz, and a few grains in the granite. 



The tourmaline occurs as light-brown prisms with an uneven 

 outline. The absorption is not so strong as is usual in tourmaline, 

 except in some grains which show traces of a fine cleavage at right 

 angles to the well-marked cross-fracture. The terminal faces are 

 rarely indicated, the prisms generally fraying out instead into light- 

 blue acicular bodies, the colour of which matches that of the inclusions 

 in the original quartz, when of similar dimensions. One very peculiar 

 feature is presented by these light-brown tourmalines : there are 

 numerous zircons enclosed in them, with pleochroic halos identical 

 with those in the biotite ; and one grain also has included the yellow 

 flaky substance mentioned in connection with that mica before. 

 Moreover, a few quartz-grains may be seen in the tourmaline. 



In the vein-quartz there are stout prisms of tourmaline; but 

 these are always blue. 



Cassiterite occurs in the greisen sparsely; the crushed rock 

 afforded a few grains showing the irregular colouring, the cleavage 

 parallel to 100, and, more rarely, a trace of the prism-faces. 



In addition to the muscovite derived from the orthoclase, another 

 mica forms small nests in the greisen : in section it cannot be 

 distinguished from muscovite ; but in a hand-specimen it resembles 

 gilbertite, a species created by Thomas Thomson 2 in 1836. 



The principal minerals produced in the greisen-area are then : 

 secondary quartz, muscovite, topaz, and brown tourmaline ; and it 

 may be safely stated that, in this case at any rate, the first three — 

 quartz, muscovite, and topaz, — have been derived from the orthoclase. 



From analyses given by Dana it is calculated that in the change 

 from orthoclase to muscovite and topaz, roughly 49 per cent, of 



1 ' Lehrb. d. Petrographie ' 2nd ed. vol. ii (1894) p. 123. 



2 ' Outlines of Mineralogy, Greology, & Mineral Analysis ' vol. i, p. 235. 



