410 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. [DoC. 1, 



headland of a yellowish sandstone, occasionally weathering red. 

 When viewed from Castle Hill, i. e, on its western side, this headland 

 presents a reddish exterior; and it is partially so externally on the 

 north and eastern faces; but when closely examined on the last- 

 mentioned flank, the rock, which rises in terrace over terrace to 

 the Lighthouse, 340 feet above the sea, is on the whole of a yellowish 

 colour. The lowest beds visible near Brough are of reddish-brown 

 colour, with blotches of light-coloured, and occasionally greenish, 

 crystalline spots ; the next ledge is a thick-bedded, finely-laminated, 

 shghtly micaceous freestone, weathering white, but partially of 

 yellowish-brown tints. The higher ' courses consist of strong bands 

 of sandstone and coarse grit of whitish, yellowish, and dull brown- 

 red colours, with rarely occurring small pebbles of quartz, — some of 

 the beds, particularly those on the summit of the headland, contain- 

 ing elongated argillaceous blotches or concretions of dark-red and 

 greenish colours*. 



These overlying sandstones are much more largely developed in 

 the Orkney Islands than in the county of Caithness. 



Old Bed of the Orkney Islands. — These islands, as far as I have 

 seen them, exhibit essentially the same geological character as the 

 rocks of Caithness. Possessing as extensive a development of the 

 ichthyolitic flagstones as Caithness itself, they exhibit a still greater 

 thickness of the superior and light-coloured sandstone, or third divi- 

 sion of the Old Eed, than is anywhere to be seen in the main land 

 of Scotland. 



In the principal island of Pomona, we observed, at two localities, 

 that the flagstones were underlaid by conglomerate and sandstone. 

 Eemarking that the beautiful Cathedral of KirkwaU was built of a red 

 freestone which has retained the sharpness of its architectural orna- 

 ments for centuries, and, seeing that the Caithness flags, on which 

 the town stands, dipped away slightly to the north, we passed over 

 eastwards to the Bay of Meal, to look out for a red sandstone of 

 which we had heard, and there found red and mottled sandstone 

 arching over and rising out from beneath the flagstones. As we 

 also detected the same mottled red sandstone on the S.E. side of 

 Scapa Bay, where it is inclined at an angle of 65° to pass under 

 the flagstone series, and trends to the westward, we had no doubt 

 that this rock, which crosses the promontory to the Bay of Meal, 

 was the same which had been worked for the construction of the 

 cathedral. This feature seems to have escaped the notice of former 

 explorers ; and the fact proves that the ichthyolite-flagstones have 

 the same base, not only of conglomerate, but also of red sandstone, in 

 Orkney as in Caithness. 



The chief masses of rock which succeed, are largely cut into in 



* Though we sought in vain for any organic remains in these yellowish sand- 

 stones, Mr. Peach has since detected plants in sandstones at Duncansby Head, 

 and also at Friswick, on the east coast. At the former place a conglomerate 

 charged with pebbles of greenstone occurs; and in one of the thin beds of 

 sandstone, a stem, apparently with leaves, was found, which is represented in 

 fig. 14 (^). 



