318 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 9, 



courses conform to the strike of the sedimentary strata, which are 

 all more or less metamorphosed. 



3. Besting upon the upturned and denuded edges of these Lower 

 Silurians occurs the Old Red Sandstone, forming a bold line of mam- 

 millated cliiFs along the southern sea board, as at Cambusmore and 

 Craig- an-airgiod, and stretching into the interior in detached moun- 

 tain-masses, as at Beinn Uarie and the Beinn-a-ghriams in Suther- 

 land, 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 sub- 

 divided 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 hgnite 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 gra- 

 nite (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| 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 (6), 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 inti- 

 mately 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 contempo- 

 raneous eflusions of plutonic rock. This granitiform rock appears, 

 at least in one instance, to traverse, across the strike, decomposed 



