BY LEO A. COTTON. 223 



is a tendency for these crystal-cavities to be arranged in planes 

 parallel to that of the vein. Several cavities and negative crystals 

 are shown in Plate xix., fig.l. Topaz is possibly present in very 

 small amount. 



Slide B. — This is distinctly more basic than slide A. ]Mica is 

 more abundant, and excellent examples of replacement of quartz 

 by this mineral are common (see Plate xix., fig. 2). Biotite occurs 

 in small amount, almost invariably showing replacement by the 

 non-ferruginous mica. A few crystals and grains of fluorite are 

 present; these are of the colourless variety, and seem to be usually 

 associated with the biotite. 



Slide C. — This slide shows a still further decrease in quartz, 

 and a corresponding increase in non-ferruginous mica. As in B, 

 a small amount of biotite is present, showing replacement by the 

 non-ferruginous mica. There is also a noticeable secondar}^ alte- 

 ration of biotite to haematite. Fluorite is more notable than in 

 Slide B, but is present only in small amount (see Plate xix. fig. 3). 

 Irregular grains of what is probably molybdenite, occur scattered 

 through the slide. 



Slide D. — This section is much the same as C, but there is a 

 still further decrease in quartz, and a corresponding increase in 

 the non-ferruginous mica. Biotite is present in small quantities 

 only; and, as before, shows replacement by the non-ferruginous 

 mica. Fluorite is present in about the same amount as in Slide 

 C, and is mainly associated with biotite. In one instance an 

 idiomorphic crystal of fluorite penetrates a biotite crystal; and, 

 along the line of junction, a development of the non-ferruginous 

 mica occurs; this particular crystal of fluorite is coloured a 

 characteristic violet. A small amount of felspar is present, and 

 two idiomorphic crystals of albite show most perfectly the inci- 

 pient metasomatic action of the non-ferruginous mica. One of 

 these is represented in Plate xix. fig. 4. 



Slide E. — One side of this slide inclines towards the normal 

 granite, and the other to the vein-material. There is, however, 

 no definite junction-line between the two extremes. The non- 

 ferruginous mica is present in large amount, possibly slightly in 



