314 Mr. MuRCHisoN on the Coal-field of Brora in Suther lands hire, 



Dunnet, Thurso, &c., consist of a moderately fine-grained red sandstone over- 

 lying- and alternating with schists, and a lamellar limestone in the adjoining 

 bays * ; the whole deposit seldom deviating from slight inclinations to the 

 east or west of North. 



The horizontal beds forming the rock of Dunnet Head, the most Northern 

 point of Great Britain, are made up of an uniform mixture of quartzose sand 

 with particles of mica, the structure distinctly stratified and the folia thin. The 

 colour is there of a brownish red, but in other places the sandstone exhibits 

 patches and stripes of bright red alternating with yellowish gray ; these mark- 

 ings are in some instances parallel to the strata, but are frequently irregular, 

 and in these particulars the specimens are almost identical with the variegated 

 or new red sandstone. It is presumed that this deposit must be of the same 

 age as the red conglomerate of the East coasts of Sutherland and Ross-shire^ 

 since the flooring-slate of Caithness, though of superior quality, is mineraio- 

 gically the same with that which is included in the red conglomerate of 

 Sutherland and Ross ; and the associated lamellar limestone throughout the 

 conglomerate of the three counties is identical, whilst the general inclination 

 of the strata is nearly the same. 



At Banniskirk, where the slate is extensively quarried, abundance of fossil 

 fish have been found for many years past ; but the attention of the Geological 

 Society was led to them by M. Culley, Esq.f, who last year sent specimens 

 from thence, I have since visited these quarries ; they are about eight miles 

 south of Thurso, and near the rise of a low range of hills, which bound upon 

 the South the extensive plains that are spread out from thence towards the 

 Pentland Frith. 



The fish occur exclusively in a schistose bed of a few inches in thickness, 

 about three feet below the uppermost stratum, and immediately overlying the 

 slate which is quarried for use. The bed containing them is rejected by the 

 workmen, and it is from the fragments scattered around the quarry that speci- 

 mens are to be obtained. When first brought to light these fish are of the 



* The most v.aluablc and extensive quarries of these Caithness slates, are at Castle Hill iti 

 Dunnet Bay (the seat of Sheriff Traill). The slate of these quarries, though chiefly composed of 

 alumine with some sliex and mica, &c., contains pyrites and also a portion of lime, as most of the 

 specimens elfervesce slowly with acids. The shale which alternates with the calcareo-siliceous 

 schists of Darrogill Bay is so highly bituminous, that it burns with a bright flame. In the 

 sandstone quarries of Thurso a much purer black bitumen is found,, generally associated with 

 crystals of carbonate of lime. 



f Mr. Culley has been for some time zealously occupied with the geology of the primitive and 

 central districts of Sutherland and Caithness ; and, 1 believe, will shortly offer to the Geological 

 Society his observations, accompanied by sections from the East to the West coast. 



