Presidential Address to Geological Section. 455 



are attenuated, there is a molecular rearrangement of the minerals, and 

 the resultant rock is a granulitic gneiss. Indeed, the evidence in the 

 field, which has been confirmed by the microscopic 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.W. and S.E. and E. and W., and the partial 

 or complete recrystallization 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 recrystallization of the rock 

 and its metamorphosis into hornblende schist. Similar results are 

 observable when the lines of movement are parallel with the course of 

 the dykes. All the stages of change from the massive to the schistose 

 rock can be traced — the replacement of the pyroxene by hornblende, the 

 conversion of the felspar, and the development of granulitic structure with 

 foliation. Here we have an example of the phenomena developed on 

 a larger scale by the Post-Cambrian movements, viz., the production of 

 common planes of schistosity in rocks separated by a vast interval of time, 

 quite irrespective of their original relations. For both gneiss and dykes 

 have common planes of foliation, resulting from earth-stresses in Pre- 

 Torridonian time. 



It is important to note also that linear foliation is developed in the 

 basic dykes where there has been differential movement of the constituents 

 in folded areas. In the case of the anticline mapped by ]\Ir. Clough, near 

 Poolewe in Ross-shire, he has shown that the linear foliation is parallel 

 with the pitch of the folds. All these phenomena tend to confirm the 

 conclusions arrived at by Mr. Teall, and published in his well-known paper 

 " On the Metamorphosis of Dolerite into Hornblende Schist." ^ 



The ultrabasic and acid rocks likewise occur in the schistose form, for 

 the peridotites pass into talcose schists and the granite becomes gueissose. 



In connection with the development of schistosity in these later 

 intrusive rocks it is interesting to observe that where the basic dykes 

 merge completely into hornblende schist, and seem to become an integral 

 part of the fundamental complex, biotite gneisses and granular hornblende 

 gneisses prevail. Whatever be the explanation the relationship is 

 suggestive. 



The unconformability between the Lewisian gneiss and the overlying 

 Torridon Sandstone, which was noted by MaccuUoch and confirmed by 

 later observers, must represent a vast lapse of time. When tracing this 

 base-line southwards through the counties of Sutherland and Ross, striking 

 evidence was obtained by the Geological Survey of the denudation of that 

 old land surface. In the mountainous region between Loch Maree and 

 Loch Broom it has been carved into a series of deep narrow valleys with 

 mountains rising to a height of 2,000 feet. In that region it is possible 

 to trace the orientation of that buried mountain chain atid the direction 

 of some of the old river courses. This remnant of Archajan topography 

 must be regarded as one of the remarkable features of that interesting 

 region. 



In 1893 the various divisions of the Torridon Sandstone, as developed 

 between Cape Wrath and Skye, were tabulated by the Geological Survey, 

 and may here be briefly summarized. They form three groups : a lower, 

 composed of epidotic grits and conglomerates, dark and grey shales 



1 Quart. Journ. Geol. Soc, vol. xU, p. 133. 



