178 BATHYMETRICAL SURVEY OF 



NOTES ON THE GEOLOGY OF THE ASSYNT DISTRICT. 



By B. N. PEACH, LL.D., F.R.S., and J. HORNE, LL.D., F.R.S. With 

 Geological Map (Plate XLIL). Published by permission of the 

 Director of the Geological Survey. 



From a geological point of view, the Assynt district is one of 

 the most interesting in the north-west Highlands. The various rock 

 formations which enter into the geological structure of the region are 

 there splendidly developed, and the evidence in proof of those great 

 terrestrial displacements of post-Cambrian date may be studied in 

 detail in the mountainous region that runs southward from Glas Bheinn 

 by Ben More Assynt and Breabag to the Cromalt Hills. 



Beginning with the Archaean gneisses (gt on map), which may be said 

 to form the foundation-stones of that region, they are unquestionably 

 older than the succeeding great development of Torridon Sandstone 

 and overlying Cambrian strata. On referring to the geological map, 

 it will be seen that they occupy a belt of ground from 6 to 9 miles broad, 

 extending along the western coast-line between Enard Bay and Stoer, 

 thence inland to the base of the grand escarpment of Torridon Sand- 

 stone that stretches southwards from Quinag to the Coigach mountains. 

 These crystalline gneisses give rise to a type of scenery that is charac- 

 teristic of a large part of the western seaboard of Sutherland and Ross, 

 which seems to be typical of Archaean areas. Bare rounded knolls and 

 bosses of grey gneiss follow each other in endless succession, and in the 

 hollows there are numerous pools and lochs occupying rock-basins. The 

 whole tract occupied by these crystalline gneisses is singularly destitute 

 of drift. The rocky knolls do not rise much above one general level, 

 which does not as a rule exceed a few hundred feet in height, save near 

 the base of Quinag, Canisp, and Suilven, where the elevation of the old 

 gneiss plateau is about 1000 or 1250 feet. 



The Archaean rocks of the Assynt district, west of the great 

 escarpment of Torridon Sandstone, consist largely of pyroxene gneisses 

 and ultrabasic rocks (pyroxenites and hornblendites), which still show 

 in a marked degree their original characters. Their behaviour in the 

 field and their appearance under the microscope have led to the 

 conclusion that they have affinities with plutonic igneous products. 

 All over that district, where the original characters have not been 

 effaced by later mechanical stresses, it is possible to trace the imperfect 

 separation of the ferro-magnesian from the quartzo-felspathic con- 

 stituents, the gradual development of mineral banding, and the net-like 

 ramifications of acid veins (pegmatite) in the massive gneiss. Whatever 

 be the origin of the mineral banding in these Archaean gneisses, it is 



