272 me. w. r. watt o~s the geology [June 19 14, 



Tourmaline-pegmatites are not uncommonly associated with the 

 later phases of igneous activity in this region. Other occurrences 

 that may he cited are near Rhyme at Craig Cailleach and west of 

 Drumallachie in Kildrummy. 



(2) Petrography. 



(a) The bytownite-norite. — As was before indicated, this 

 type, covering the largest area, undergoes several modifications of 

 texture and mineral composition. Dark in colour and of medium 

 grain, the typical norite is composed of hypersthene and basic 

 plagioclase, together with monoclinic pyroxene, brown hornblende, 

 biotite, and olivine ; while small amounts of magnetite, pyrite, 

 apatite, and zircon are found. Microscopic examination proves the 

 texture to be markedly ophitic. 



The hypersthene possesses negative birefringence, strong pleo- 

 chroism, and many schiller inclusions, while its irregularly-bounded 

 plates are frequently intergrown with a monoclinic pyroxene. The 

 latter may also form a core to a hypersthene shell, or a ring round 

 a hypersthene core, or exist as diallage. 



The felspar of the rock was determined from its mean refractive 

 index. The Becke test being used, a liquid of the refractive index 

 of felspar-flakes was obtained by mixing together monobrornnaph- 

 thalene and dimethylaniline. The refractive index of the liquid 

 was then determined by an Abbe reflectometer for each mixture. 

 In this case the average of several determinations gave ^i=l"567: 

 this would indicate a felspar the composition of which lay between 

 Ab 2 An 3 and Ab T An 4 — that is, a somewhat acid bytownite. The 

 alteration of the felspar is slight, and little schillerization occurs. 

 Twinning on both albite and pericline laws is very common. 



Olivine, usually in small rounded grains, is nearly always 

 present in varying amount. Slight alteration to green or brown 

 serpentine may occur, while reaction-rims between the olivine and 

 the felspar are occasionally present. As in the olivines of the 

 gabbro type, the rims are composed of two minerals, here too 

 small for certain identification. The outer pale-green pleochroic 

 mineral is probably actinolite. 



Brown hornblende is widespread, though not common. It may 

 crystallize out in parallel growth with the pyroxene, surrounding 

 that mineral like a fringe. Behaving like the pyroxene, it may 

 be ophitic to the plagioclase, or enclose small olivine-grains ; while 

 it is often found as a border to small grains of magnetite. 

 Similarly, biotite is of general occurrence, being most abundant in 

 the neighbourhood of Bin Quarry. As it forms here especially 

 along joints, its presence may be due to the later biotite-bearing 

 central intrusion. It is strongly pleochroic in A'elloAv, chocolate- 

 brown, and red-broAvn. 



The only other noritic type in this part of Scotland that is 

 knoAvn to me occurs near Rhynie in a small stream — the Broom 

 Stripe — on the north side of the serpentine-mass of Towanreef. 



