1858.] MUECHISON NOETHEEN HIGHLANDS, ETC. 403 



quantity of bitumen increases to so great a degree as to exude from 

 cracks and fissures in the stone, the quality of the paving- or floor- 

 stone necessarily diminishes ; but when the value of this bitumen is 

 sufficiently known, the flagstones permeated by it may be largely 

 quarried for lighting and other purposes. 



Eevisiting that portion of the north coast of Sutherland which 

 confines upon Caithness, and which was formerly described by 

 Professor Sedgwick and myself, I found that, whilst the lower part 

 of the Old Red series in the tract extending from Strathy to 

 Reay exhibits in many places (and particularly near Port Skerry) 

 shore -ledges well exposed at low water, of powerful coarse breccia 

 made up of granite, porphyry, and the flaggy gneiss which is pene- 

 trated by these rocks, this coarse breccia, including huge angular 

 blocks from 2 to 3 feet in size, is, notwithstanding many breaks, 

 observed to be surmounted by sandstone of lightish colour, which, 

 though it weathers to a brownish red colour, cuts, as a freestone, 

 to a whitish tint. The thick-bedded sandstone with pink grains is 

 surmounted at BaUigill by dark-grey ichthyoHte -flags charged with 

 Coccosteus, Osteolepis, Dijpterus, Diplopterus, Cheir acanthus, and cer- 

 tain plants. These beds graduate into limestones which have been 

 extensively worked for use, one of the beds being 3 feet thick and 

 followed by thin flaggy courses. On the whole, however, the lower 

 part of the series in its western extension is already seen to be 

 parting with the character which is dominant in Caithness, and is 

 becoming more a sandstone formation, with included limestone. 



The bosses of the granite on the shore near Port Skerry have evi- 

 dently penetrated, when in a molten state, into the gneiss, whilst 

 both these rocks have afforded the fragments composing the lower 

 breccia and conglomerate. In truth, all this part of the coast having 

 been subjected to violent breaks and faults, the hard crystalline 

 granite, porphyry, altered rocks, and gneiss have in such movements 

 been necessarily protruded here and there through the softer sand- 

 tone and flagstone. But that all such crystalline rocks were pre- 

 viously solidified, is proved by the fact that their materials forming 

 the breccia and conglomerate here constitute the highly irregular 

 base of the Old Red series. This angular breccia, evidently formed 

 under tumultuous conditions, alternates, however, with fine-grained, 

 hard, siliceous sandstones, into which it passes up, and finally into 

 building-stone. 



Pig. 12. — Section along the Burn of Isauld, near Reay, showing a 

 passage from the Lower to the Middle Member of the Old Bed Series. 



N. by E. S, by W. 



Sea-coast. Beay. 



* Granite, a. Granitic red conglomerate, b. Hard grey siliceous sandstone. 

 c. Calcareous dark-coloured flagstone, with geodes of hmestone. d. Soft light- 

 coloui-ed sandstone, e. Ordinary dark-grey Caithness flags, with ichthyolites. 



