180 BATHYMETRICAL SURVEY OF 



Beag and of the Coigach mountains beyond. Though at the base there 

 is sometimes a local breccia that varies in character in accordance with 

 the underlying rocks, this pile of sediment mainly consists of a 

 succession of false-bedded grits and sandstones, with scattered pebbles 

 derived from formations which do not now occur in the west of 

 Sutherland and Ross. On Quinag and Beinn Gharbh the sandstones 

 have a gentle dip to the south of east, but on Suilven the strata are 

 horizontal, or nearly so. They attain a thickness of several thousand 

 feet, for in the Coigach mountains they rise from the shores of Loch 

 Broom to a height of about 2400 feet. 



Overlying the Torridon Sandstone come the various subdivisions of 

 the Cambrian formation, comprising the basal quartzite (a 1 on map), 

 pipe-rock (a 2 ), fucoid beds (a 3 ), serpulite grit and limestone (a 4 ). The 

 detailed mapping of that region has proved that the Cambrian strata 

 are separated from the Torridon Sandstone by a marked uncon- 

 formability, It represents an interval of time during which the 

 Archaean floor and overlying Torridonian sediments were exposed to 

 denudation; a vast thickness of strata was removed, and in places the 

 Archaean gneisses were laid bare. Hence we find in the undisturbed 

 area clear evidence of the double unconformability of the Cambrian 

 quartzites on the Torridon Sandstone and Archaean gneiss. This 

 important geological feature is well displayed on the north slope of 

 Beinn Gharbh, south of Loch Assynt. The age of these sediments 

 has been proved by the discovery of trilobites and other organisms, 

 characteristic of the lower division of the Cambrian system, in the 

 fucoid beds of Sutherland and Ross. Fragments of these trilobites 

 have been found in this member of the series at BLnockan and on the 

 north shore of Loch Assynt. 



On referring to the map, it will be seen that to the west of the 

 band of limestone (a 4 ) extending from Inchnadamph to Knockan, the 

 Cambrian quartzites and fucoid beds have been traced across the sheet 

 from Loch Gainmheich to Strath Kanaird. On the eastern slopes of 

 Quinag, Canisp, and Cul Mor, the white quartzites form a thin cake 

 on the underlying Torridon Sandstone, which on some of the lofty 

 peaks is isolated by denudation. The quartzites dip at a higher angle 

 than the sandstone, and on descending the hill slopes the former pass 

 transgressively across bed after bed of the sandstone, and rest succes- 

 sively on lower members of the Torridon Sandstone. 



One of the remarkable features of the Assynt district is the series 

 of intrusive igneous rocks of later date than the Cambrian limestone 

 and older than the post-Cambrian movements. In the undisturbed 

 area west of the great post-Cambrian displacements, they cover con- 

 siderable areas on Beinn Gharbh, south of Loch Assynt, where they 

 appear as sills in the Torridon Sandstone or Cambrian quartzite. 

 These sills can be traced round the western slopes of that hill, as well 



