1873.] JTJDD THE SECONDARY ROCKS OF SCOTLAND. 117 



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 con- 

 sisting of the Lower division of the Old Eed 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 

 tip 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 de- 

 position 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 

 flariks, 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 mis- 

 taken 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 sur- 

 faces of which are decomposed and stained with oxide of ironf . 

 When, however, a sufficiently large surface of fracture can be ob- 

 tained, 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 Jvilcallumkil (Gordon Bush) a great mass of Old Eed 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 gra- 

 dual retrocession of the glacier of Strath Brora, and one of which still dams up 

 the present lake. Professor Gcikio has referred to this most interesting locality 

 in his admirable book on the Scenery of Scotland, p. 203-4. 



t 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." 



I See Cunningham, ' Geognosy of Suthcrlandshire,' pi, vii. 



