144 TOURMALINE-BEARING ROCKS, MT. HEEMSKIRK. 



The Normal Granite. 



The rock which we have termed the normal granite con- 

 sists of a medium to coarse-grained biotite granite. It is 

 much more widely distributed than the other types, and 

 appears to be of more uniform composition. The rock shows, 

 on microscopic examination, the following minerals : — 

 Orthoclase, plagioclase felspar, biotite, quartz, tourmaline, 

 iron pyrites, and apatite. The felspars have suffered much 

 decomposition ; the clouded appearance of the plagioclase 

 felspar makes the determination of its character very diffi- 

 cult, but the low angle of extinction points strongly to its 

 being oligoclase. It has preceded the orthoclase in the order 

 of crystallisation. The biotite is very pleochroic, and con- 

 tains on the whole very few inclusions ; it appears to have 

 undergone very slight alteration, but it is occasionally 

 somewhat bleached, and a small amount of resorption with 

 separation out of opaque material has taken place. Apatite 

 in small grains is rare, and small masses of iron pyrites, 

 destitute of crystal boundaries, are present. The main 

 interest of the slides centres round the remaining minerals, 

 quartz and tourmaline. The tourmaline occurs as short 

 slender opaque rods, traversing the quartz grains in all 

 directions. The rods, as a rule, are quite straight, but in 

 some few instances they are curved; a radiant structure is 

 very characteristic, the centre of radiation being sometimes 

 a minute speck of opaque m.atter ; in other cases the rods 

 appear to radiate from the line of separation of two quartz 

 grains, and the rods then show a tendency to lie parallel 

 to this line of separation ; no case was observed of a rod 

 crossing the line of separation and penetrating both of two 

 adjacent quartz grains. 



The distribution of the rods in the quartz Is very capri- 

 cious ; they crowd some grains and are comparatively rare 

 in others, while sometimes the same grain may be rich in 

 them in one part while the remainder of the grain is abso- 

 lutely destitute of them. In one marked case a quartz 

 grain . is crowded with needles, and carries a moderate 

 amount of minute opaque specks, while the remainder of 

 the grain, though in optical continuity, is devoid of rods, 

 and almost devoid of opaque specks. The phenomenon 

 would appear to point to the presence here of quartz of two 

 generations, the younger quartz having been deposited 

 from a solution free from tourmaline, and having crystal- 

 lised round an eroded grain of the older quartz in optical 

 continuity with it. The presence of veins containing iron 

 pyrites in the vicinity seems to support the hypothesis that 



