388 EECEls'T WORK OE THE GEOLOGICAL SURVEY 



2. Unfoliated Igneous BocJcs in original Gneisses. 



A remarkable feature of these original gneisses is the occur- 

 rence among them of numerous masses of highly basic igneous rocks 

 (gabbros, peridotites, paleeo-picrites, pyroxene-granulites, and 

 diorites), either possessing no foliation, or foliation of such an im- 

 perfect type that it is impossible to tell its angle of inclination. 

 They occur as lenticular zones or belts running for several 

 hundred yards more or less parallel with the foliation, or as 

 irregular patches covering about a quarter of a square mile of 

 ground. Many of the dark bands of gabbro or diorite are highly 

 garnetiferous (pyroxene-granulite), the garnets having no special 

 arrangement except in those cases where they have been affected by 

 later Pre-Cambrian movements. 



These patches of non-foliated igneous rock are intersected by 

 veins of grey pegmatite varying in thickness from a few inches to 

 several yards, consisting mainly of felspar and quartz usually 

 opalescent. Occasionally a small quantity of pyroxene or horn- 

 blende is associated with the quartz and felspar. In some instances 

 the pegmatites are so prominently developed that they form a large 

 proportion of the mass, and in such cases it frequently happens that 

 " eyes " or bosses of the basic rock have been isolated from the 

 parent sheet. Where the basic rocks are non-foliated, the peg- 

 matites have no regular arrangement; where, on the other hand, 

 incipient foliation is displayed in the former, the latter are drawn 

 out parallel with the direction of movement. 



In tracing the boundaries of many of these non-foliated masses 

 and their pegmatites, it is observable that the dark eruptive rocks 

 pass gradually into the rudely foliated basic gneisses ; while the 

 pegmatites merge into the grey highly quartzose bands, consisting 

 mainly of opalescent quartz and felspar. The conclusion is, 

 therefore, obvious that the original types of gneiss in the west of 

 Sutherland have been formed out of eruptive basic rocks and the 

 pegmatites developed in them prior to the foliation. It is equally 

 apparent also that the relative proportion of pegmatite and other 

 segregation-veins to basic rock in the non-foliated areas should 

 generally correspond with the proportion of grey highly quartzose 

 gneiss to the more basic varieties. 



3. Evidence of mechanical Movements in the Formation of the 

 original Gneisses. 



A careful study of the coast-sections at Lochinver and Kylesku 

 reveals the fact that these original gneisses possess certain structures' 

 analogous to those met with in the quartz-schists overlying the 

 fossiliferous limestones and quartzites produced by the Post-Lower- 

 Silurian movements. The planes of schistosity traverse the various 

 basic rocks and pegmatites, irrespective of the boundaries of the 

 original materials. Such a result could not have been produced if 

 the foliation had been due to the deformation of a mass of half 

 consolidated Plutonic rock at the time of the intrusion. The 



