THE FRESH- WATER LOCHS OF SCOTLAND. 181 



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 Ruathair. 

 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 aegirine. 



Before proceeding to the description of the eastern or Moine schists 

 (in 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 



