274 ME. W. E. WATT OX THE GEOLOGY [June 1914, 



hornblende, in which last cases the magnetite may represent original 

 olivine. 



In the neighbourhood of Bin Hill — round the Elfhouse, for 

 example — an olivine -gabbro occurs, which is marked by an increase 

 in the amount of pyroxene and a corresponding diminution in the 

 amount of felspar. 



In the troctolite and pic rite types, the dominant minerals are 

 felspar and olivine, while the picrites contain in addition pyroxene 

 and amphibole, as well as more abundant olivine. Among the 

 troctolites two modes of weathering can be recognized: — (1) where 

 the green serpentinized olivine occupies depressions among the 

 resistant felspars ; and (2) where the felspars have succumbed, and 

 reddish olivines are found in relief. The latter type of weathered 

 rock found near Broomhillock admirably displays the linear 

 arrangement of the constituents. 



In addition to the minerals mentioned above, biotite, apatite, 

 magnetite, and pyrite are found. Serpentine (green or brown) is 

 produced in weathering, and, between olivine and felspar, rims of 

 anthophyllite or enstatite and actinolite occur. 



In basicity, the felspar varies from rock to rock, being more acid 

 in the more acid rocks. Thus, in the troctolites of Bin Hill 

 and Inchstammack, where there is more felspar than olivine, the 

 mean refractive index of 1*564 would indicate a composition of 

 Ab 2 An 3 ; whereas the banded rock from Craighead Quarry has a 

 felspar whose mean refractive index of 1'570 indicates a bytownite 

 approximating to A^An^ 



In the troctolite forming part of the Rothiemay intrusion the 

 production of rims of fibrous amphibole has been greatly developed, 

 and the remaining- felspar is either in part, or completely, altered 

 to prehnite. 



In the more basic types, lustre-mottling is well seen, due to the 

 poecilitic inclusion of the olivines in pyroxene, amphibole, biotite, 

 or felspar. Of the last, fine specimens are found in Sinsharnie 

 Quarry, where the white decomposing felspar meanders among 

 dark grains of serpentine. Though not arranged with any optical 

 constancy, the olivine-grains often show a parallel elongation. 

 Towards the felspars reaction-rims are found. The felspars are in 

 this case basic, with a mean refractive index of 1'570. 



The chief pyroxene of the rock is augite, the rock thus being 

 an augite -picrite. The augite becomes altered to a brown opaque 

 mass, which pcecilitically encloses serpentine-grains. At the same 

 time, the felspars, besides being invaded by fibrous amphiboles, are 

 largely altered, with the production of prehnite. With regard to 

 the olivine — hypersthene, brown hornblende, and biotite behave in 

 a similar fashion. The biotite is strongly pleochroic in straw- 

 yellow and reddish-brown, and at the same time possesses a high 

 birefringence. Besides magnetite and pyrite, mention must be 

 made of pleonaste and chromite. 



The Kothiemay intrusion is similar to the others, although here 

 the main pyroxene is hypersthene. The felspar is not very abundant, 



