THE ERESH-WATER LOCHS OF SCOTLAND. 
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as round the escarpments of Suilven and Canisp. But in the displaced 
masses, east of a line extending from Inchnadamph to Knockan, the 
intrusive rocks of this series have a much larger development and 
greater variety. They appear at intervals over a tract measuring 12 
miles from north to south, and from 5 to 6 miles from east to west. 
The largest of these masses extends from Ledmore and Cnoc na Sroine 
eastwards by Aultnacallagach towards Cnoc Chaoruinn, and another 
important sheet runs north from Loch Ailsh to Loch Sail an Huathair. 
But throughout the mountainous region of Glas Bheinn, Ben More 
Assynt, and Breabag these igneous rocks appear as sills in the various 
thrust-masses, restricted generally to certain definite horizons. A 
glance at the map will show that they occur at the base of the Cambrian 
quartzite, in the basal quartzite, in the pipe-rock, in the fucoid beds, 
and also in the limestone. The mapping of these intrusive sheets has 
shown the complicated character of the geological structure of that 
region. The petrographical characters of these igneous materials have 
been studied by Mr. Teall, and are of special interest. They comprise 
the plutonic mass of Cnoc na Sroine and Loch Borralan, and the 
numerous sills and dykes that traverse the Torridonian and Cambrian 
sediments. The former seems to have resulted from the consolidation 
of alkaline magmas rich in soda ; at the one end of the series there is 
the quartz-syenite of Cnoc na Sroine, and at the other the basic augite- 
syenite, nepheline-syenite, and borolanite. The sills and dykes include 
two well-marked types — viz., hornblende-felspar rocks, and felsites with 
alkali felspar and segirine. 
Before proceeding to the description of the eastern or Moine schists 
(m on map), reference must be made to those terrestrial movements 
which affected that region in post-Cambrian time, whereby the 
Cambrian rocks were piled on each other, and huge slices of the floor 
of Archaean gneiss with the overlying Torridonian and Cambrian 
sediments were driven westwards and made to override the underlying 
piled-up strata. The structure is admirably shown in the horizontal 
section extending from Quinag to the river Cassley, placed below the 
map, showing the surface geology of the Assynt district. On referring 
to that section, it will be seen that at its western limit on Quinag, 
where the rocks are undisturbed, the Torridon Sandstone rests on a 
highly eroded platform of Archaean gneiss, being itself unconformably 
overlaid in turn by the Cambrian quartzites, fucoid beds, and serpulite 
grit (3, 4, 5, and 6 in section). In the valley of the Skiag, north of 
Loch Assynt, the first disruption line or thrust-plane is met with, above 
which lie various members of the Cambrian system, chiefly the fucoid 
beds, serpulite grit, and limestone, with their accompanying intrusive 
sheets of igneous material, all of them being driven together by minor 
thrusts or reversed faults or folds. 
Crossing the limestone plateau at Achumore to the western base 
