ol-l JVofices of Meiiioir>i — British Associafioii — 



Complex. They are intimately associated with each other, and have 

 a common foliation. The muscovite-biotite-gneisses (1) occur together 

 with the biotite-gneisses, but they are not abundant. The pyroxene- 

 gneiss (5) is recorded only from one locality, viz. Dalb;eag, near 

 Carloway, where it forms part of a basic mass which is cut by the 

 foliated granite of Carloway (9). The hornblende-schists (6) con- 

 stitute basic masses in the complex, with a foliation more or less 

 parallel with that of the contiguous gneisses. The relative age of 

 the members of this group has not been definitely ascertained. The 

 pegmatites and pegmatite-gneiss (8) intersect the other components of 

 the complex, but they are sparingly represented compared with the 

 great development of these types on the mainland between Laxford 

 and Cajie Wrath. One example of pyroxenite (7) was obtained in 

 the policies of Stornoway Castle. Mylonites (10) are typically developed 

 in certain sections along the eastern seaboard between Tolsta Head and 

 Loch BhroUum. 



In the areas examined, the north-west and south-east strike, referred 

 to by Murchison, is not characteristic of the gneisses of Lewis. It 

 is prevalent immediately to the west of Stornoway, but exceptional in 

 other tracts. In this connexion the observations of Professor James 

 Geikie in the Eye peninsula were confirmed. The dominant strike over 

 extensive areas runs a few degrees east of north and west of south ; 

 in certain localities it is north-east asd south-west, and in others nearly 

 east and west. 



The prevalent types of gneiss closely resemble those to be found on 

 the mainland between Loch Laxford and Cape Wrath, without the great 

 series of acid intrusions. The structure is coarsely granular, or 

 granulitic, the mineral grains being rounded and not elongated. The 

 range of rock types seems to be comparatively limited, for there is 

 a marked absence, in the areas examined, of the pyroxene-gneisses with 

 blue quartz, of pyroxene-granulites, and other basic and ultrabasic 

 materials, which are so characteristic of the Fundamental Complex 

 between Lochinver and Scourie on the mainland. The remarkable 

 series of basic and ultrabasic dyke intrusions in the west of Sutherland 

 has not been detected in Lewis. 



The flaggy granulitic gneisses of the Butt of Lewis which appear to 

 run soutliwards along the belt of high ground between Stornoway and 

 Barvas are of special interest. In structure they closely resemble the 

 Moine gneisses east of the Moine thrust-plane, but they differ petro- 

 logically from the rocks of sedimentary origin that form the Moine 

 Series of the Geological Survey. The system of over-folding and the 

 direction of the axial planes of the folds approximate to those found 

 in the Moine rocks on the mainland. 



The platy rocks or mylonites, noted by Macculloch, 03cur along 

 definite lines of movement, trending a few degrees east of north and 

 west of south. Actual thrust-planes have been detected, which are 

 inclined to the south of cast at low angles, as if the displacements had 

 been in a westerly direction. Various stages in the development of 

 mylonites from the acid and basic gneisses are represented. 



VII. — The Origin of Kopjes and Inseleerge.^ By J. D. Falconer, 



M.A., D.Sc. 



DETACHED hills, projecting crags, and i olated rocks arc feature? 

 of almost every landscape, and in the moister regions of the globe 

 their origin has usually been correctly assigned to the ordinary processes 

 of denudation. They may arise either through dissection of earlier 



' Abstract of paper read before the British Association, Section C (Geology), 

 Dundee, September, 1912. 



