the Primarj/ Rocks and the Oolitic Series in the North of Scotland. 1 H 



stone^ pyritous shale^ sandstone, &c. &c. The siliceous beds give the type 

 to the lower part of the formation, and the calcareo-biluminous bed to the 

 intermediate part — again becoming more siliceous and arenaceous at the 

 upper portion, and so appearing- to graduate into the superior division. 3rdly. 

 A great formation of red, brown, and variegated sandstone, which on the 

 opposite side of the Pentland Firth reappears in the lofty red clifls of the 

 Orkneys, and there also, as in Caithness, reposes upon a calcareo-bituminous 

 schist. 



The aggregate thickness of these deposits is enormous : and their original 

 extent was probably much greater than it is at present ; as it, perhaps, once 

 filled up a great hollow or trough of the primary rocks, which, towards the 

 north, are seen in the Shetlands and some of the Orkneys, and, towards the 

 south, rise into the mountains of Sutherland. 



Fossil Fish of the Caithness Schist. 



When the attention of geologists was first drawn to these ichthyolites, it 

 was not known that specimens of them were to be found in any other quarry 

 than that of Banniskirk. The authors of this memoir have, however, since 

 discovered that similar remains are extensively, it may even be said universally, 

 spread over the Caithness deposit; and that their occurrence is not confined 

 to one particular stratum, but is characteristic of this vast schistose formation 

 from the highest to the lowest beds. In the superior beds, the fish, or frag- 

 ments of them, occur abundantly near Howburn Head, north of Thurso, and 

 also at various places along the shores of the Pentland Firth ; on the north side 

 of which, beds containing the same fish are prolonged into the opposite islands 

 of the Orkneys. 



In the transverse section across the interior of Caithness*, these ichthy- 

 olites were successively met with at Widel slate quarries, three miles south- 

 east of Thurso, at Banniskirk, at Clythe and Lybster, and finally at Latheron 

 Wheel near the base of the formation. The fish occur invariably in beds of 

 dark grey calcareous schists, highly bituminous and micaceous, which beds 

 at Banniskirk and at most of the above localities immediately overlie the 

 best and largest roofing-slate. In general, the animal remains are easily dis- 

 tinguished from the matrix by their darker colour; but, at Banniskirk, they 

 are also remarkable in changing their hue, upon exposure to the atmo- 

 sphere, from the usual dark grey to a purple plum colour, as formerly le- 

 markedf. By chemical analysis, which was kindly undertaken by Mr. 

 Herschel, it appears, as might be expected, that the ichthyolites dilfcr from 



* See Plate XIV. fig. 2, f See Geol. Trans. Second Scries, vol. ii. Pari II. p. 314—31.0. 



