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 Boss. 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^ on map), 
pipe-rock fucoid beds serpulite grit and limestone (a^). 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 Boss. Fragments of these trilobites 
have been found in this member of the series at Knockan 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 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 
