the Primary Rocks and the Oolitic Series in the North of Scotland. 133 



district between Reay Kirk and Forse ; and good roofing slate is quarried at a short distance south 

 of the latter place. The summit of Howburn-head, a promontory N.N.W. of the town of Thurso, 

 consists of very fissile and nearly horizontal beds of sandstone flag and schist, with a slight dip to 

 the north ; and, in the more calcareous and slaty portions, are remains of fish, and abundance of 

 black, brilliant, quadrangular scales. Much pyrites occurs in this part of the formation, especially 

 in small concretions upon the surface of the flag beds, so that upon exposure in the dykes they 

 acquire from decomposition a ferruginous and yellow tinge, although of a blue colour when freshly 

 quarried*. In descending from Ilowburn.head to the town of Thurso, lower beds of white and 

 greenish white sandstone are quarried ; and between the lamina; of these, small portions of black 

 bitumen are sometimes interposed, and frequently associated with calcareous spar. East of 

 Thurso, the shore is composed of a number of low reefs of the calcareous flag. beds, which, at 

 Howburn-head, have been mentioned as occupying the summit of a cliff of considerable elevatioi\ : 

 the dip is still north 10°. Advancing towards Murkle Bay, the top of the clift' is composed of 

 fine, calcareous grit alternating with calcareous shale. In the lower part, is a green sandy mica- 

 ceous slate-clay, covered with pyritous markings at the separation of the beds, and the dip follows 

 the flexures of the headlands changing to N.N,E. and E,N.E. These changes are not effected 

 without several breaks, which run nearly transverse to the bearing of the strata. Under all the 

 preceding are blue, calcareous flag -beds, containing fish-scales, &c. They are, at first, of an in- 

 termediate mineralogical character ; but, following tliera in the ascending order, they gradually 

 conform to a new type, and, from their mode of weathering and false bedding, as well as from the 

 blotches and red streaks with which many of them are coloured, they may be compared with 

 some of the harder and lower divisions of the new red sandstone. In our attempts at classifica- 

 tion, we depend very little upon these characters, which apply with equal precision to many 

 varieties of the old red sandstone series. 



Still further to the east is a small bay, where the strata are in great confusion ; but, on the 

 whole, they dip towards the north. Beyond this, the calcareous grit and green pyritiferous shales 

 occur as before, and are succeeded by the calcareous, blue flagstone, with impressions of fish. 

 There is again a great break in the strata, and several indications of a concretionary structure. 

 The dip varies to the E. of N. ; and, at the slate quarry of Castle Hill, it is N.E. by N, ; so that 

 the whole system of these flag beds is evidently carried under the red sandstone of Dunnet.head, 

 the most northern promontory of Scotland f. The lowest beds of that great headland are seen 

 beyond the eastern shore of Dunnet Bay, in which the intervening strata are buried under blown 

 sand. The upper beds of this headland, which rises to the height of about six hundred feet, are 

 also composed of red sandstone, with a slight inclination to the north ; and their general character 

 has been described in a previous memoir in the Geological Transactions by one of the authors of 

 this paper +. 



At the point of junction between the calcareous flagstone and the red sandstone on the eastern 



* This change of colour is very characteristic of the fissile beds throughout large districts, and 

 seems in all cases to be due to the decomposition of the small nodules of pyrites which coat over 

 the calcareous flagstones when they are first raised in the quarries. 



t This very valuable slate quarry is described in a former memoir: Geol. Trans. Second Series, 

 vol. ii. Part II, p, 314, 



+ Although it is not proposed on this occasion to enter into any geological description of the 

 Orkney Islands, yet it may be stated that those magnificent headlands which lie uiion the nortlKrn 

 side of the Pentland Firth are composed of the same red sandstone as that of Dmniet Head ; 

 whilst, in many of their lower bays, the Caithness schists and flagstones reappear from beneath 

 the sandstone. For the memoir referred to above, see Geol. Trans. Second Series, Part II. 

 p, 314. 



