38 STKATIGRAPHICAL GEOLOGY 



parts] as well-banded gneisses in which the constituents have a 

 distinct parallel arrangement. Both varieties are traversed by 

 segregation veins and pegmatites. The prominent minerals are 

 plagioclase felspar, pyroxene (augite), hornblende, quartz (frequently 

 opalescent), and magnetite. It is worthy of note that mica is a rare 

 constituent of these original types of gneiss. On close examination 

 it is apparent that the bands present certain lithological varieties 

 of variable thickness ; some consisting mainly of pyroxene or 

 hornblende and a small quantity of plagioclase felspar ; some of 

 plagioclase, pyroxene or hornblende, and opalescent quartz ; others 

 of opalescent quartz and felspar. These varieties frequently cross 

 the lines of schistosity, and are evidently due to differences in the 

 nature of the materials prior to the development of the foliation." 



In some places these foliated rocks include masses of non-foliated 

 basic igneous rock (gabbros, peridotites, augite -granulites, and 

 diorites), arranged in lenticular belts which are more or less 

 parallel to the foliation. These patches and belts are intersected 

 by veins of grey pegmatite, consisting mainly of felspar and 

 quartz, and varying in thickness from a few inches to several 

 yards. But when these non-foliated masses are carefully traced it 

 is found that they pass laterally into the rudely foliated basic 

 gneisses and that the pegmatites merge into quartz-felspar schists. 

 Hence it is concluded that almost the entire mass consisted 

 originally of igneous rocks, and that the gneisses have been 

 developed by deformation alone ; but that there was an original 

 differentiation of the felspathic and hornblendic elements, either by 

 separate intrusion or by fluxion, while the rocks were still in. a 

 viscous condition. There is no doubt, however, that the members 

 of the original complex must have been subsequently affected by 

 intense pressure inducing plastic deformation and metamorphism. 



The gneisses are also traversed by other dykes of basic igneous rock 

 diabase, epidiorite, and picrite but these are nevertheless of a 

 date anterior to that of the overlying Torridon sandstone which they 

 do not enter.- Most of these dykes run from S.E. to N. W. or W.N.W. 

 Moreover, detailed mapping of the area has shown that after the 

 intrusion of the igneous dykes, and still before the deposition of the 

 Torridonian, the whole region was subjected to intense lateral 

 pressure which caused much plication of the gneisses and produced 

 bands of disruption or "shear-zones" which run approximately 

 east and west. 



In certain parts of the area, i.e. near Gairloch and Loch Maree, 

 rocks of a different character make their appearance and seem to lie 

 within a broad and deep synclinal fold of the Hebridean gneiss. 3 

 They consist of schists which are clearly altered sediments, but their 



