IN THE NORTH-WEST HIGHLANDS OF SCOTLAND. 385 



characters of some of the thrust Hebridean gneisses in Assynt and 

 at Ullapool which show indications of crushing and recementation. 

 In some instances these features have so obscured the original 

 structure that it is difficult to determine the true characters of the 

 rocks. 



Subsequently Dr. Callaway referred to certain localities where 

 the members of the Silurian series become more highly altered 

 towards the junction with the Archaean gneiss, when the latter, by 

 folding or thrust, has been made to overlie the former. He main- 

 tained that there is no material alteration in the Silurian series 

 underlying the Hebridean gneiss in Glencoul, because there is no 

 evidence of extraordinary pressure ; but near the base of the Stack 

 of Glencoul, at the junction with the eastern gneiss (Caledonian), 

 the quartzite loses all traces of clastic structure and passes into 

 quartz-schist. He accounts for this progressive alteration by 

 enormous pressure due to the quartzite being " reflexed again and 

 again in closely adpressed folds"*. 



The investigations of Professor Lapworth demand special notice, 

 because they involve a departure from Professor Nicol's views 

 regarding the nature and origin of the eastern schists of Sutherland. 

 Selecting the region of Durness and Eriboll, he mapped a large 

 portion of it in great detail during the summers of 1882 and 1883. 

 In the pages of the ' Geological Magazine ' he published a series of 

 papers on " The Secret of the Highlands "f, in which he described 

 the geological structure of that region, completely confirming Nicol's 

 conclusions (a) that the Durness Limestone is the highest member 

 of the " Ordovician " series (Lower Silurian, Murchison), (b) that the 

 " Upper Quartzite " and " Upper Limestone " are non-existent, (c) 

 that there is no conformable sequence from the quartzites and lime- 

 stones into the eastern gneissic series, (d) that the line of junction 

 of the unaltered Palaeozoic rocks is a line of fault and overthrust. 

 But the results of his work, in so far as they affect the age, com- 

 position, and mode of formation of the eastern schists, were read at 

 a meeting of the Geologists' Association, July 4th, ]884J. As 

 these results are practically identical with those obtained in- 

 dependently by the Geological Survey, and published in the official 

 Report (' Nature/ vol. xxxi. p. 39), it is desirable to give a brief 

 summary of them : — 



1. The lithological distinctions between the Hebridean gneiss and 

 the Logan and Arnaboll rocks are primarily due to the mechanical 

 disturbances to which the latter have been subjected. 



*■ " The Limestones of Durness and Assynt," Quart. Journ. GeoL Soc. vol. 

 xxxvii. p. 239. 



"The Torridon Sandstone in Eelation to the Ordovician Rocks in the 

 Northern Highlands," Quart. Journ. Geol. Soc. toI. xxxviii. p. 114. 



" The Age of the Newer Gneissic Rocks of the Northern Highlands," Quart. 

 Journ. Geol. Soc. vol. xxxix. p. 355. 



" Notes on Progressive Metamorphism," Geol. Mag. dec. 3, vol. i. no. 5, p. 218. 



t Geol. Mag. dec. 2, vol. x. pp. 120, 193, 337. 



I Proc. Geol. Assoc, vol. viii. p. 438 ; see also Geol. Mag. dec. 3, vol. ii. 

 p. 97 (1885). 



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