456 British Association — J. Home, F.R.8., etc. — 



with calcareous bands, red sandstones, and grits ; a middle, consisting 

 of a great succession of false-bedded grits and sandstones ; an upper, 

 comprising chocolate-coloured sandstone, 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 Sand- 

 stone 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 

 unconformably 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 Con- 

 glomerates 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 

 contact alteration, black and yellow cherts, jaspers with spherulitic 

 structures which indicate that they have been formed by the silicification 

 of liparites of the ' Lea-rock ' type and spherulitic felsites that bear 

 a striking resemblance to those of Uriconian age in Shropshire. These 

 interesting relics have been derived from formations which do not now 

 occur anywhere in the western part of the counties of Sutherland and 

 Ross, and they furnish impressive testimony of the denudation of the 

 Archaean plateau in Pre-Torridonian time. 



These Torridonian sediments, like the sandstones of younger date, 

 contain lines of heavy minerals, such as magnetite, ilmenite, zircon, and 

 rutile.3 The dominant felspar of the arkose group is microcline, that of 

 the basal group oligoclase. In the calcareous sediments of the upper and 

 lower groups fossils might naturally be expected, but the search so far has 

 not been very successful. Certain phosphatic nodules have been found in 

 dark micaceous shales of the upper group which have been examined by 

 Mr. Teall. From their chemical composition these nodules might be 

 regarded as of organic origin ; but he has found that they contain spherical 

 cells with brown-coloured fibres, which appear to be debris of organisms.* 



Early in last century the Torridonian deposits were referred by Mac- 

 culloch ^ and Hay Cunningham " to the ' Primary Red Sandstone,' and by 

 Murchison,'' Sedgwick, and Hugh Miller to the Old Red Sandstone. The 

 structural relations of the Torridon Sandstone to the overlying series of 

 quartzites and limestones were first clearly shown by Professor Nicol,® 

 who traced the unconformability that separates them for 100 miles across 

 the counties of Sutherland and Ross. When Salter pointed out the 

 Silurian facies of the fossils found in the Durness Limestone by Mr. Charles 

 Peach, the Torridonian formation was correlated with the Cambrian rocks 



1 Geol. Mag., Dec. Ill, Vol. VII (1890), p. 516. 



2 Ann. Eep. Geol. Surv., 1895, p. 20. 



3 Ann. Eep. Geol. Surv., 1893, p. 263. 



* Ibid., 1899, p. 185. 



* Trans. Geol. Soc., set. i, vol. ii, p. 450 ; " The "Western Isles of Scotland," 

 vol. ii, p. 89. 



6 Trans. Highland and Agricultural Society of Scotland, vol. xiii (1839). 

 ' Trans. Geol. Soc, ser. ii, vol. iii, p. 155. 



* Quart. Journ. Geol. Soc, vol. xiii, p. 17. 



