LAKES IN RELATION TO GEOLOGICAL FEATURES 441 
chocolate-coloured sandstones, micaceous flags, dark shales and 
calcareous bands. The total thickness of this great pile of sediment- 
ary deposits must be not less than 10,000 feet. 
The Torridonian strata can be traced from the precipitous head- 
lands of Cape Wrath to Applecross, where they form pyramidal 
mountains of great height. They reappear to the east of this undis- 
turbed area, among the displaced masses affected by the post-Cambrian 
movements, notably in Assynt, in the area extending from Kishorn to 
Loch Alsh, in Sleat, and in Rum. 
CAMBRIAN 
The Torridon Sandstone is overlain unconformably bv an im- 
portant series of fossiliferous strata comprising quartzites, fucoid 
shales, Salterella grit, dolomites, and limestones. The age of these 
sediments has been definitely fixed by the discovery in the fucoid beds 
of trilobites belonging to the Olenellus zone — the lowest division of 
the Cambrian system. In the neighbourhood of Durness these 
dolomites and limestones reach their greatest development, and are 
there divisible into zones, some of which have yielded cephalopods, 
gasteropods, lamellibranchs, brachiopods, and sponges. It seems 
probable that the greater part of the Durness limestone represents 
the middle and upper divisions of the Cambrian system, and possibly 
the base of the Lower Silurian rocks of North America. 
An interesting series of plutonic igneous rocks, ranging in com- 
position from quartz-syenite to nepheline-svenite and borolanite, 
appear in the Cambrian strata of Assynt, which are accompanied by 
numerous sills and dykes comprising felsites, porphy rites, and 
vogesites. These intrusions are later than all the Cambrian rocks of 
the region, and older than the post-Cambrian movements. 
The fossiliferous Cambrian strata are followed eastwards, and in 
certain sections are visibly overlain by a great development of 
crystalline schists, which Murchison regarded as conformable with 
the underlying sediments. But this theory of natural sequence was 
not accepted by Professor Nicol, who contended that the superposition 
of the Eastern schists on the Cambrian rocks was due to earth- 
movements. The detailed examination of the region by Bonney, 
Lapworth, Callaway, and the Geological Survey has confirmed 
the accuracy of NicoFs main conclusions. For, by means of lateral 
compression or earth-creep the strata have been thrown into a series 
of inverted folds which culminate in reversed faults or thrusts, the 
effect of which is to bring lower over higher beds. This reduplication 
of the strata by inverted folds and reversed faults is an accompaniment 
of the great horizontal displacements by which thick slices of Lewisian 
Gneiss, Torridon Sandstone, Cambrian rocks, and the Eastern Schists 
