372 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. [DoC. 1, 



No one who has advanced from the west to the east, and left 

 behind him the old gneiss, can for a moment fail in recognizing the 

 distinction between this overlying crystalline flagstone and the old 

 gneiss, though it also contains felspar as well as quartz and mica : 

 it is not hornblendic and massive, and is void of granitic veins. In 

 other localities, to be considered presently, it will be seen how the 

 limestone is directly superposed by a band of pure white quartz - 

 rock. One of the best detailed sections, showing the succession of 

 the limestone, is seen to the north of the House of Balnakeil, and 

 in some of the northernmost headlands, places in which Mr. Peach 

 coUeeted many of his fossils. Previous to my last visit, I had 

 supposed (and Professor Nicol had published a section to show it*) 

 that the Durness limestone was abruptly cut oif by the old gneiss 

 upon the east, between Durness and Loch Eribol. But such is not 

 the case ; for Mr. Peach had observed, and I confirmed his observa- 

 tion, that between the interior ridge of old gneiss which extends 

 from the western foot of Ben Keannabin and the limestone of Dur- 

 ness, the dip is reversed, and the underlying quartz-rock is again 

 brought up, lying between the gneiss and the hmestone, as repre- 

 sented in the section, fig. 3, p. 364. 



The reversal by which the Durness limestone is thus placed in a 

 trough of quartz -rock and overlying hmestone has been manifestly 

 occasioned by a great upheaval of the old gneiss when acted upon by 

 eruptive forces, of which clear signs are manifested in the adjacent 

 Bay of Sangoe. There huge bosses of black hornblendic and hyper- 

 sthenic rock stand out with serpentinous coatings, — the courses of 

 the hmestone in their vicinity being singularly altered, mottled and 

 dolomitic. 



Again, as we ascertained that Parred or Far-out Head consisted 

 of the old gneiss, there is now no doubt that the limestone and under- 

 lying quartz-rock af Durness occupy a trough. The clearest proof of 

 the trough-shaped arrangement of the strata is a httle to the west, 

 or inland, from the well-known limestone promontory in which the 

 large caves of Smo occur. This limestone, which dips very slightly 

 seawards or N.N.E., is highly altered, sihceous, and prismatized by 

 a number of vertical lines of rude cleavage, which cause the rock to 

 spht into innumerable brittle fragments. On leaving the head- 

 land, and on mounting to the low-peaked hill of the quartz-rock 

 called Sangoe Beg, certain white cherty and sihceous beds are re- 

 markable as graduating into the lower quartz -rock, which is seen to 

 rise gradually from imder the limestone until the beds attain the 

 high angle of 65°, the dip to the K.IS'.E. being traversed by the same 

 rude vertical cleavage. A deep gully here alone separates the quartz- 

 rock from the older gneis». 



Quitting the basin of Durness, including the large limestone island 

 of Hoan, and passing across the noble longitudinal marine loch of 

 Eribol, the geologist who proceeds to the E.S.E., or towards Ben 

 Hope or Loch Tongue, has thenceforward a clear and unmistake- 



* Quart, Journ. Geol. Soc. Tol.xiii. p. 23. fig. 4. 



