THE FRESH- WATER LOCHS OF SCOTLAND. 183 



post-Cambrian displacements that affected that region, the outcrop 

 of which is somewhat remarkable. On referring to the geological map, 

 it will be seen that it can be traced from Loch nan Caorach and Gorm 

 Loch Mor east of Glas Bheinn, southwards along the eastern base of 

 the Ben More group of mountains to Loch Ailsh, thence across the 

 Oykell to the Cromalt hills. Here the outcrop of the thrust-plane 

 changes its course, and runs west along the base of these hills to 

 Knockan, a distance of 6 miles, whence it runs southwards to Strath 

 Kanaird. It will thus be seen that there is an extraordinary overlap 

 of the Moine thrust-plane along the base of the Cromalt hills, for it 

 passes transgressively across the Ben More thrust-plane and all 

 underlying thrusts till the materials overlying it rest directly on the 

 undisturbed Cambrian strata south of Knockan. 



Near the Moine thrust the new structures resulting from the post- 

 Cambrian movements are well developed. The lenticles of Lewisian 

 gneiss and pegmatite are sheared and rolled out, the former passing 

 into flaser gneiss and schist, and ultimately into a banded platy schist, 

 while the latter show fluxion structure with felspar " eyes " like 

 rhyolites. The Torridon Sandstone and Cambrian quartzites, the 

 fucoid beds and intrusive igneous sheets, are likewise sheared and 

 rolled out, the new divisional planes being more or less parallel to that 

 of the Moine thrust. Indeed, such is the transformation effected by 

 these movements on the crystalline rocks and overlying sediments, 

 that it is often difficult to determine the original characters of the 

 component members. It is noteworthy, however, that all the crushed 

 or mylonised rocks near the Moine thrust show a characteristic striping 

 on the divisional planes due to orientation of the constituents in the 

 direction of movement. 



The strata overlying the Moine thrust-plane and stretching east- 

 wards down the Cassley and the river Oykell and across the Cromalt 

 hills are remarkably uniform in character. They consist to a large 

 extent of flaggy quartzose schists, with partings and bands of mica- 

 schists and occasional intrusive sheets or sills of igneous material which 

 have a common foliation with the schists. The matrix of the quartz- 

 schists is holo-crystalline and forms a granulitic mosaic, which is 

 perhaps the characteristic feature of the group. Occasionally "eyes" 

 of felspar appear in the schists, when the rocks might be described as 

 flaser schists. There can be little doubt that the Moine schists are to 

 a large extent, if not wholly, altered sediments, the age of which is 

 still uncertain. Any one who has examined the Archaean rocks in 

 the undisturbed area west of the Torridon Sandstone escarpment, has 

 no difficulty in distinguishing the pyroxenic gneisses and intrusive 

 dykes from the quartz-schists and mica-schists of the Moine series. 

 These broad lithological distinctions have been of great service in 

 interpreting the history of the glaciation of that region. 



