618 REPORT — 1901. 



are parallel with the course of the dykes. All the stages of change from the 

 raassive to the schistose rock can be traced — the replacement of pyroxene by 

 hornblende, the conversion of the felspar and the development of grariulitic 

 structure with foliation. Here we have an example of the phenomena developed 

 on a larger scale by the post-Oambrian movements, viz., the production of common 

 planes of schistosity in rocks separated by a vast interval of time, quite irrespec- 

 tive 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 difterential movement of the constituents in folded areas. In 

 the case of the anticline mapped by Mr. 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 pub- 

 lished in his well-known paper ' On the Metamorphosis of Dolerite- into Hornblende 

 Schist.' 1 



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

 peridotites pass into talcose schists and the granite becomes gneissose. 



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 horn- 

 blende 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 Macculloch and confirmed by later observers, must 

 represent a vast lapse of time. When tracing this base-line southwards through the 

 counties of Sutherland and Eoss, striking evidence was obtained by the Geological 

 Survey of the denudation of that old land siirfiice. 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 and the direction 

 of some of the old river courses. This remnant of Archaean 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, which may here 

 be_ briefly summarised. They form three groups : a lower, composed of epidotic 

 grits and conglomerates, dark and grey shales with calcareous bands, red sand- 

 stones, and grits ; a middle, consisting of a great succession of false-bedded grits 

 and sandstones; an upper, comprising chocolate-coloured sandstones, micaceous 

 flags with dark shales and calcareous bands. The total thickness of this great pile 

 of sedimentary deposits must be upwards of 10,000 feet, and if Mr. Clough 's 

 estimate of the development of the lower group in Skye be correct, this amount 

 must be considerably increased. Of special interest is the evidence bearing on the 

 stratigraphical variation of the Torridon Sandstone when traced southwards across 

 the counties of Sutherland and Ross. The lower group is not represented in the 

 northern area, but southwards, in Ross-shire, it appears, and between Loch 

 Maree and Sleat varies from 500 to several thousand feet in thickness. These 

 divisions of the Torridon Sandstone are of importance in view of the correlation of 

 certain sediments in Islay with the middle and lower Torridonian groups which 

 there rest imconformably on a platform of Lewisian gneiss. 



In continuation of the researches of Dr. Hicks, published in his paper ' On Pre- 

 Cambrian Rocks occurring as Fragments in the Cambrian Conglomerates in Britain,' ^ 

 Mr. Teall has specially investigated the pebbles found in the Torridon Sandstone. 

 The local basement breccias of that formation have doubtless been derived from the 

 platform of Lewisian gneiss on which they rest, but the pebbles found in the coarse 

 arkose tell a different story .^ He has found that they comprise quartzites showing 



' Quart. Journ. Oeol. Soc, vol. xli. p. 1.S3. 



= Geol. 3Tag., 1890, p. 616. 



' Annual Report of the Geological Survey for 1895, p. 20. 



