Vol. 59.] GEANITE AND GEEISEN OF CLIGGA HEAD. 149 



greisen, show obscure traces of crystalline outline, remain apparently 

 intact in the altered rock, and can be easily distinguished with the 

 naked eye. 



Numerous minute, dark, crystals of tourmaline occur throughout 

 the granite and the greisen. 



In order to study the transition from granite to greisen, as well 

 as the weathered state of the former admits, slices were prepared 

 from a series of specimens taken from the hollow mentioned on p. 147, 

 starting from the granite and continuing to the quartz-vein in 

 the centre of the greisen-band. The microscopic characters 

 of the component minerals will first be described as briefly as 



The felspar in the granite, where distinguishable, is completely 

 kaolinized ; but the cleavage, as so often happens in decayed felspar, 

 remains very distinct. For the most part, it has been replaced by 

 minute flakes of muscovite with ragged outlines, and by 

 secondary quartz, which occasionally shows an indistinct hexa- 

 gonal form. In the greisen a trace of kaolinized felspar may be seen 

 near the granite, though otherwise it is completely altered to secondary 

 quartz and muscovite. 



But for the absorption, the biotite, of which there is very little 

 in the greisen, would be completely colourless ; as it is, it shows a 

 faint vandyke-brown, which is more pronounced in flakes parallel 

 to the basal plane. The pleochroic halos, surrounding zircons and 

 other, but undeterminable, inclusions are as perfect as those in 

 fresh biotite, and by reason of the bleaching much more distinct. 

 ^Magnetite occurs as a decomposition-product; and one biotite-erystal 

 shows the commencement of a sagenite-web. A grain of brown 

 tourmaline is sometimes included. Another, yellow, flaky, and 

 presumably decomposition-product was observed in one grain. The 

 axial figure is characteristic. 



The original quartz, both in the greisen and the granite, forms 

 large grains with few, but well-developed, fluid-cavities. On the 

 periphery of almost every grain, however, the 

 Fig. 4. — Prism fluid-inclusions are much more numerous and 

 of tourmaline slightly smaller than those in the centre. One 

 included in or two minute zircons and one tourmaline-crystal 

 quartz. are seen embedded in the clearer quartz, but 



near the quartz-vein, where it becomes somewhat 

 difficult to distinguish between the original and 

 secondary quartz. But the most remarkable in- 

 clusions are numerous long prisms with terminal 

 faces, sometimes lying with no particular arrange- 

 [x about 800 ment, though often orientated to the prism and 

 diameters.] pyramid-planes. By their form and colour, which 



is faint blue, they can only be referred to tourma- 

 line. The average length of these inclusions is 0*16 millimetre ; but 

 one stouter prism, the form of which instantly suggests tourmaline 

 (fig. 4), measures only 0*032 millimetre. That these prisms are 

 actually embedded in the original quartz admits of no doubt. 



