Lying upon the upturned and denuded edges of these Silurian rocks there is, on the south-east coast of Sutherland, a series of outlying masses, forming a belt about five miles wide, and consisting of the Lower division of the Old Red Sandstone system. This is constituted by nearly horizontal beds of the well-known and highly remarkable rock of the Old Red Conglomerate (which is made up of fragments of all sizes, waterworn and angular, of the subjacent Silurian rocks) alternating with, and frequently graduating into, more or less flaggy beds of Red Sandstone composed of what Sir Roderick Murchison aptly calls "granitic sand." Nothing can be more striking than the proofs of unconformity between the Silurian and Old Red Sandstone rocks : the former have evidently been not only contorted and metamorphosed, but also upheaved and denuded before the deposition of the latter ; and the older strata have, moreover, furnished the materials of which the younger are composed.
The relation of these two series of rocks may be well seen on both sides of Loch Brora, where the tops of the fantastically shaped mountains, which culminate in Beinn-Smeorail and Beinn-Hourn, are formed of the Old Red Conglomerate and Sandstone, while their flanks, wherever mountain -torrents have cut through the old lateral glacier-moraines which cover them, are seen to be formed of the highly contorted Silurian rocks*. On the south-eastern side of the band of the Old Red strata the Silurian distinctly appears ; but the rock, being of a somewhat peculiar character, was originally mistaken for granite. It is almost entirely made up of quartz and felspar, and is generally in a more or less altered condition, being divided by numerous joints into small angular fragments the surfaces of which are decomposed and stained with oxide of iron†. When, however, a sufficiently large surface of fracture can be obtained, the laminar arrangement of the crystalline materials of the rock is perfectly manifest. Mr. Cunningham states that a rock of precisely similar character forms part of the series of strata, now recognized as Silurian, at Beinn-Laoghal and some other points in the interior of Sutherland ; and fragments of the rock certainly occur as pebbles in the Old Red Conglomerate.
In the admirable section exposed in the ravine near Clyne Kirk‡ the Silurian rocks, which rise to the height of about 500 feet (at which elevation their greatly contorted strata are seen to be capped by the nearly horizontal Old Red Sandstone beds) , terminate abruptly;
- Near Kilcallumkil (Gordon Bush) a great mass of Old Red Sandstone has
tumbled from the mountain above nearly to the level of the Loch ; and in it a quarry has been opened. Nowhere can the geologist find better illustrations of glacial phenomena than on the shores of the exquisitely beautiful Loch Brora. Of especial interest are the numerous terminal moraines, which mark the gradual retrocession of the glacier of Strath Brora, and one of which still dams up the present lake. Professor Geikie has referred to this most interesting locality in his admirable book on the Scenery of Scotland, p. 203-4.
† This rock affords an admirable material for macadamizing roads, as it falls when quarried into suitable angular fragments, without needing the labour of the " stone-breaker."
‡ See Cunningham, ' Geognosy of Sutherlandshire,' pl, vii.
