TRANSACTIONS OF SECTION C. 617 



limestones and dolomites -with tremolite, garnet and ep:dote.^ 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 alHuities with 

 plutonic igneous products have not been satisfactorily determined. B\u the 

 detailed mapping has proved that north of Loch Maree they rest ou a platform of 

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

 Riabhach), and that both the gneiss complex and altered sediments have been 

 aifected by a common system of folds. In the iield, bands of mylonised rock have 

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

 examination of tbe latter by Mr. Teall has revealed cataclas^tic 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 serie.** 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 chissitication 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 (pendotite),_(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, whtre 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 difficult to separate from tbe 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 horn- 

 blende gneisses with which they are associated. 



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

 pubjected to enormous terrestrial stresses which profoundly ati'ected the funda- 

 mental 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 

 Kyle.sku, the gneiss loses its low angle, is thrown into sharp folds, the axes of 

 which are parallel with the planes of movement. The folia are attenuated, there 

 is a molecular rearrangement of the minerals, and the resultant rock is a granulitic 

 gneiss. Indeed, the evidence in the held, which has been confirmed by the micro- 

 scopic examination of the rocks by Mr. Teall, seems to show that granulitic biotite 

 and hornblende gneisses are characteristic of the zones of secondary shear. A 

 further result of these earth-stresses is the plication of the original gneisses in 

 sharp folds, trending N.AV. and S.E. and E. and W. ; and the partial or complete 

 recrystallisation of the rocks along the old planes of mineral banding. 



in like manner, when the basic dykes are obliquely traversed by lines of 

 disruption, they are deflected, attenuated, and within the shear zones appear 

 frequently as phacoidal masses amid the reconstructed gneiss. These phenomena 

 are accompanied by the recrystallisation of the rock and its metamorphosis into 

 hornblende schist. Similar results are observable when the lines of movement 



' Annual Report of the Geological Survey for 1895, p. 17. 

 » Hid., p. 18. 



