530 H. S. JEVONS, H. I. JENSEN, T. G. TAYLOR AND C. A. SUSSMILCH. 



classed in two distinct groups : the pegmatitic which is 

 coarser than the normal rock, and the aplitic which is of 

 equal or finer grainsize. There are two kinds of pegmat- 

 ites, and three kinds of aplites, distinguished from one 

 another by their grainsize. All grade into one another, 

 but the types named are the most abundant. The finer 

 pegmatite has a variable grainsize, approximately 2*5 mm. 

 and is thus but little coarser than the main rock. The 

 coarser varies considerably in its grainsize, roughly from 

 5 to 10 mm., but individual crystals more than 30 mm. in 

 length are not uncommon. Towards the southern end of 

 the Reservoir Quarry the pegmatite of the great segre- 

 gation veins contains abundant ilmenite in exceptionally 

 large crystals. They are thin hexagonal flakes, sometimes 

 skeleton-like, a deep rich brown in colour, with almost the 

 splendent lustre of biotite, and commonly 3 or 4 cms. in 

 diameter, though in one place they reach 10 cm. in greatest 

 diameter. It is curious that they seem to stand roughly 

 perpendicular to the edge of the vein. 



The finest-grained aplite is almost compact in appearance, 

 its grainsize varying from 0*1 to 0*15 mm. In the medium 

 variety the grainsize varies from 0*25 to 0*75 mm., and in 

 the coarse aplite from 1 to 2 mm. In different patches of 

 the same kind of aplite, or from part to part of one patch, 

 the grainsize may vary between the limits stated (which 

 do not pretend to great accuracy); but the boundaries 

 between aplites of different grainsize are always extremely 

 sharp. The coarse aplite often has the same grainsize as 

 the main rock of the intrusion, but is quite distinct both 

 from it and from the fine-grained pegmatite in texture and 

 composition. 



The most interesting feature of the segregation veins is 

 the mutual relations of these different varieties of aplite 

 and pegmatite when they occur side by side in the same 



