1858.] MUECHISON NOETHEEN HIGHLANDS, ETC. 401 



gical composition which these rocks assume in different parts of their 

 range, or attempt to describe their numerous undulations as well 

 as the powerful fractures to which they have been subjected. Li- 

 thologicaUy described many years ago by Sedgwick and myself, and 

 since then treated of in my own publications*, they have acquired a 

 wide celebrity for their great economical value, whilst they are deeply 

 interesting to the palaeontologist as containing those numerous ich- 

 thyolites described by Hugh Miller and Agassiz. 



The real study of these flagstones is best made upon the coast- 

 cliffs, from the lowest strata near Dunbeath to their central parts at 

 Wick, and thence to the highest beds on the shores of the Pentland 

 Firth, where they pass up into the overlying sandstones of Dunnet 

 Head (fig. 9, p. 394). Many of the beds are so bituminous, owing to 

 the quantity of animal matter they contain, that even in 1827 they 

 were described by me as bituminous schists f. 



One of the most instructive ascending sections in the environs of 

 Thurso is made (as pointed out to me by Mr. Robert Dick) by 

 proceeding from the gneissose rocks of the Dorrery Hills on the 

 W.S.W. to Thurso and Holborn Head on the E.N.E. In this district 

 the observer proceeds from the crystalline rocks through a vast 

 thickness of superposed strata. The lowest of these, as usual, are 

 breccias and conglomerate, followed by various sandstones in parts 

 calcareous ; and then follows the flagstone series, in which Mr. Dick 

 enumerates from 13 to 15 varieties, including sandstones, one variety 

 of which, at the old Bishop's Castle near Thurso, is used as a build- 

 ing-stone. This flagstone series is extended into the various head- 

 lands of the north coast, and is prolonged eastwards in broken 

 undulations to Dunnet Head, Duncansby Head, &c. Until a correct 

 map of the coimty be constructed, it is difficult, if not impossible, 

 so to lay down the range of the strata as to show those parts where the 

 flagstones expand to vast dimensions, and where they are affected by 

 powerful and numerous fractures and faults J ; for, though the outline 

 of the country is on the whole one of gently sloping bills and plains, 

 the fractures to which the strata have been subjected are powerful 

 and numerous. Many of these dislocations are well exhibited in 

 the coast- cliffs ; and according to Mr. Robert Dick they are equally 

 abundant inland, and particularly around LochWatten. Hence it is 

 very difficult to form an estimate of the thickness of the flagstone 

 group. There are, however, tracts (as between Lochs Rangan and 

 Stemster, and again between the Dorrery Hills and Banniskirk) where 



* See particularly the last edition of ' Siluria,' in wliich the Chapter on the 

 " Old Red Sandstone " was printed long before these memoirs were read. 



t Trans. Greol. Soc. Lond. 2nd ser. vol. ii. p. 314. 



I Spreading out a heap of flour upon the board of his bakery, Mr. Robert 

 Dick, who had heard me complaining of the want of any map of Caithness, pro- 

 duced, in a short time, a model, in reUef, of the ground and drainage of this 

 county, the geography of which (with the exception of the excellent charts of 

 the coast made by the Admiralty Sui'veyors) is in a worse state than in any part 

 of Scotland. I blessed the Duke of Sutherland for having had a good map of 

 Sutherland executed, whilst the want of any map of Caithness approaching to 

 accuracy is sorely felt by the exploring geologist. 



2 g2 



