448 Dr. J. W. Gregory — Moine Pebbles 



Dr. Home, 1 in his address to the British Association in 1901, quoted 

 the authority of Dr. Teall and Dr. Peach for the resemblance of 

 altered Torridon Sandstone to the Moine ; and he again remarked this 

 resemblance in the memoir on the North- West Highlands. 2 The late 

 W. Gunn went further, and in the same work claimed (p. 612) that 

 "east of Dundonnell good evidence can be adduced that altered 

 Torridon Sandstone has entered largely into the composition of the 

 Eastern schists". The recent memoir on the Fannich Mountains 

 represents some of the flaggy granulites of that district as due "to 

 the crushing of Torridon Grit ". 3 



As the Moine and Torridonian rocks both originated as quartzo- 

 felspathic sediments, the fact that they give rise to similar rocks 

 under the influence of dynamo-metamorphisjn does not prove that 

 they are of the same age. The claim that the Moine rocks are 

 altered Torridonian rests upon two main arguments. First, the 

 absence of the Torridon Sandstone from the eastern side of the 

 overthrusts, whereas, if it were a later formation, some outlier upon 

 the Moine Series might be expected. The weight of this argument 

 is lessened by the facts that between Loch Alsh and Glenelg typical 

 representatives of the Moine Gneiss and of the Torridon Sandstone 

 are faulted against one another, and that the Cambrian rocks also are 

 absent from the surface of the Moine Series in the area east of the 

 overthrust band. 



The second and weightier argument rests on the view that no 

 pebbles of the Moine rocks occur in the Torridon conglomerates ; and 

 if this fact were confirmed it would support the view that the 

 Torridon Sandstone is not a later formation than the Moine. 

 In the Survey Memoir on the North- Western Highlands, Dr. Teall's 

 ■chapter on the petrography of the Torridon Sandstone contains 

 an account of the included pebbles (op. cit., p. 283); he points 

 ■out that schistose and highly metamorphic rocks are rare in 

 the Torridonian conglomerates except in the basal breccias, which 

 are full of Lewisian fragments. He mentions seven or eight pebbles 

 from the Torridon Sandstone in the Survey collection, of which all 

 but two or three are quite different from the Moine rocks'; the others 

 .•are " mainly composed of quartz showing marked signs of dynamic 

 action", and these characteristics give no adequate clue to the origin 

 •of these pebbles. Hence the Survey memoir appears to support the 

 absence of Moine rocks from the Torridonian conglomerates. So also 

 -does the more recent memoir on Sheet 92 (1913, p. 40), which gives 

 a list of pebbles in the Torridonian beds, including "grit, quartzite, 

 ■chert, jasper, felsite, and quartz porphyry, which are quite unknown 

 in the Lewisian gneiss " ; but it does not mention any rock that was 

 regarded as even probably Moine. 



1 J. Home, 1901. " B'ecent Advances in Scottish Geology": Eep. Brit. 

 Assoc, 1901, p. 622. 



3 The Geological Structure of the North-West Highlands of Scotland (Mem. 

 Geol. Surv. Great Britain, 1907), p. 468. 



3 The Geology of the Fannich Mountains and the Country around Upper 

 Loch Maree and Strath Broom (Sheet 92, Mem. Geol. Surv. Scotland, 1913), 

 p. 81. 



