318 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9,
courses conform to the strike of the sedimentary strata, which are
all more or less metamorphosed.
3. Resting upon the upturned and denuded edges of these Lower Silurians occurs the Old Red Sandstone, forming a bold line of mammillated cliffs along the southern sea board, as at Cambusmore and Craig-an-airgiod, and stretching into the interior in detached mountain- masses, as at Beinn Uarie and the Beinn-a-ghriams in Sutherland, Mor-bheinn, the Smians and Maiden-pap in Caithness, and Suidhe-'n-fhir-bhig on the county march. These Old-Bed-Sand- stone conglomerates underlie brownish gritty beds, passing upwards at Dornoch into thick-bedded, light-coloured sandstone, where the occurrence of Holoptychian scales and fucoid plants suggests their belonging to the upper part of the middle zone of the series as subdivided by Sir B. Murchison.
Along the coast, from Golspie to the Ord of Caithness, rocks of Liassic and Oolitic age occur, chiefly as low skerries or reefs, much flexed, and with a general northerly dip, but conforming occasionally in their curves to the present contour of the coast, as they dip at high angles off its headlands or border its bays. At Clyne these rocks pass from the Lias of Dunrobin, with its Hippopodium ponderosum and other characteristic fossils, into thick-bedded siliceous sandstones of Oolitic age, some of whose very numerous fossil forms are believed to be nearly allied to those of the Greensand. Here glacial striae from N.W. to S.E. are beautifully preserved on the highly indurated and almost cherty sandstone. Below this occurs the lignite known and once worked as the Brora coal, while to the eastward, as at Kintradwell and Culgower, Plesiosaurus has recently been found, together with a new species of the genus Gyrodus, now named by Sir Philip De M. G. Egerton Gyrodus Goweri.
The associated igneous rocks are a large-grained porphyritic granite (a) ranging along the coast from near Allt-choille on the south to the burn of Ousdale on the N.E., a distance of about 15 miles, with a breadth of 3-1/2 miles at Kil-Pheadar on the river Ullie or Helmsdale. This rock forms the mountain-mass of the Ord of Caithness, at which point, and at Culgower, it has been found to contain a considerable quantity of purple fluorspar. True granites and syenites, with bosses of hornblendic rock and greenstone, occur towards the west in Strathfleet, a variety most abundant in the auriferous district being a red small-grained granite (b), generally associated with beds more or less micaceous, to the strike of which its main courses conform; while it occasionally sends out across the strata, as if into transverse fissures, short dykes from which small veins insert themselves between the micaceous beds as along lines of least resistance (fig. 1). In the most richly auriferous localities, certain granitoid rocks, chiefly felspathic (c), are so intimately connected by interlamination with the flaggy quartzose strata, that they almost appear to be the result of metamorphic action upon true sedimentary rocks of the quartzose series, or contemporaneous effusions of plutonic rock. This granitiform rock appears, at least in one instance, to traverse, across the strike, decomposed
