45i British Association — J. Some, F.E.S., etc. — 



it is certain that they possessed this banding, and were thrown into gentle 

 folds before the uprise of the later intrusive dykes. 



The crystalline schists that have afl&nities with rocks of sedimentary 

 origin occupy limited areas north of Loch Maree and near Gairloch. The 

 prominent members of this series are quartz schists, mica schists, 

 graphitic schists, hmestones and dolomites with tremolite, garnet and 

 epidote.i They are there associated with a massive sill of epidiorite and 

 hornblende schist. The relations which these altered sediments bear to 

 the gneisses that have af&nities with plutonic igneous products have not 

 been satisfactorily determined. But the detailed mapping has proved 

 that north of Loch Maree they rest on a platform of Lewisian gneiss, and 

 are visibly overlain by gneiss with basic dykes (Meall Eiabhach), and 

 that both the gneiss complex and altered sediments have been affected 

 by a common system of folds. In the field, bands of mylonized rock 

 have been traced near the base of the overlying cake of gneiss, and the 

 microscopic examination of the latter by Mr. Teall has revealed cataclastic 

 structures due to dynamic movement. It is obvious, therefore, that, 

 whatever may have been the original relations of the altered sediments 

 to the gneiss complex, these have been obscured by subsequent earth- 

 stresses. 



The great series of later igneous rocks which pierce the fundamental 

 complex in the form of dykes and sills is one of the remarkable features 

 in the history of the Lewisian gneiss. In 1895 Mr. Teall advanced 

 a classification of them,^ but his recent researches show that they are 

 of a much more varied character. For our present purpose we may omit 

 the dykes of peculiar composition and refer to the dominant types. These 

 comprise : (1) ultrabasic rocks (peridotite), (2) basic (dolerite and epidiorite, 

 and (3) acid (granite and pegmatite). The evidence in the field points 

 to the conclusion that the ultrabasic rocks cut the basic, and that the 

 granite dykes were intruded into the gneisses after the eruption of the 

 basic dykes. The greater number of these dykes consists of basic 

 materials. It is important to note that the basic rocks best preserve 

 their normal dyke-like features in the central tract between Scourie and 

 Lochinver, where they traverse the pyroxene gneisses. But southwards 

 and northwards of that tract, in districts where they have been subjected 

 to great dynamic movement, they appear as bands of hornblende schist, 

 which are difl&cult to separate from the fundamental complex. The acid 

 intrusions are largely developed in the northern tract between Laxford 

 and Durness ; indeed, at certain localities in that region the massive and 

 foliated granite and pegmatite are as conspicuous as the biotite gneisses 

 and hornblende gneisses with which they are associated. 



After the eruption of the various intrusive dykes the whole area was 

 subjected to enormous terrestrial stresses, which profoundly affected the 

 fundamental complex and the dykes which traverse it. These lines of 

 movement traverse the Lewisian plateau in various directions, producing 

 planes of disruption, molecular rearrangement of the minerals, and the 

 development of foliation. It seems to be a general law that the new 

 planes of foliation both in the gneiss and dykes are more or less parallel 

 with the planes of movement or disruption. If the latter be vertical or 

 nearly horizontal the inclination of the foliation planes is found to vary 

 accordingly. 



Close to the well-defined disruption planes, like those between Scourie 

 and Kylesku, the gneiss loses its low angle, and is thrown into sharp folds, 

 the axes of which are parallel with the planes of movement. The folia 



1 Ann. Eep. Geol. Surv., 1895, p. 17. 

 » Ibid., p. 18. 



