134 H. S. JEVONS, H. I. JENSEN AND 0. A. SUSSMILCH. 



the pegmatite and essexite except that it is usually coarser 

 grained. These pegmatites do not appear to have been 

 formed by any kind of pneumatolytic action ; the absence 

 in them of pneumatolytic minerals and their texture dispel 

 any such idea. They are then, not altogether comparable 

 with such of the granite-pegmatites which have had a 

 pneumatolytic origin. 



The aplite is invariably finer grained than the essexite 

 and is always more or less miarolitic. Three varieties, 

 according to grainsize, are shown in Table II, but at any one 

 particular place the aplite usually consist of two types (a) 

 fine and (b) coarse. The former is fairly uniform in 

 character and is usually aphanitic (microcrystalline); 

 the latter is very variable in grainsize and phanero- 

 crystalline, the grainsize in both cases tending to increase 

 with the thickness of the vein. The fine grained type of 

 aplite occurs in the large veins either as (a) a definite band 

 on either side of the coarse aplite, (b) as irregular rounded 

 patches in the coarse aplite or (c) filling the centre of the 

 vein to the exclusion of the coarse aplite. The coarse 

 always occurs, more or less, in the middle of the vein, 

 sometimes as a mere thread (see Pig. 2) sometimes as a 

 network of irregular veins traversing the fine aplite or even 

 the pegmatite, at places swelling into large masses, 

 sometimes disappearing altogether. 



The coarser aplites are strongly miarolitic and contain 

 in many places fairly large elongated cavities. Where the 

 coarse aplite occurs as a thin vein in a finer-grained aplite 

 as shown in Plate II, it is strongly miarolitic, and there is 

 a tendency for the felspar tables to stand approximately 

 at right angles to the walls of the vein. Such thin veins 

 may have been formed, by pneumatolytic action. In their 

 appearance and structure they resemble the thin segre- 

 gation veins of the Bostonite of Bowral (N.S.W.). 



