On the Geological Structure of the HigJilands of Scotland. 35 



Sir R. I. Murchison,* following on the discovery by Mr. Charles 

 Peach, of Lower Silurian fossils in the Durness and Assynt 

 limestone (1854). Previous to that period the crystalline 

 schists of the Central Highlands were considered to be of " pri- 

 mary " age ; and the red sandstones and conglomerates of the 

 north-west coast were supposed to be of the same geological age 

 as those of the north-east coast — namely of " Old Red Sandstone " 

 age. 



Sir R. I. Murchison demonstrated that the red sandstones and 

 conglomerates of the west lie beneath all the crystalline schists, 

 quartzites, and limestones of the Central Highlands, to which they 

 are unconformable,t and as the latter are shown, by the fossils 

 found in the Assynt (or Durness) limestone, to be of Lower Silurian 

 age, the underlying sandstones are inferred to be representatives 

 of the Cambrian, and the fundamental gneiss and schists, which 

 imderlie the Cambrian, are also inferred to be representatives of 

 the Laurentian group of Canada, which lies at the base of the 

 Cambrian and Silurian rocks of that country. 



The general section may be stated as follows : — 



Formations op the Northern Highlands. 

 Lower Silurian Beds, 'probably from 7,000 to 10,000yeei. 



Approximate 

 thickness 

 in Feet. 



(a.) Gneiss, qnartzite, mica-schist passing down into the 



Upper Qnartzite of Benmore in Assynt, . (over) 5,000 



(6.) Ribband glossy slates and flagstones, . . (abont) 500 



(c.) Assynt limestone — upper part yellowish and dolomitic 

 in places ; lower white, and full of cavities like casts 

 of fossils, but rarely with fossils themselves, . . 100 to 1 000 



id.') Ferruginous sandy flags, shales, and bands of limestone, 



containing '' fucoid " markings, . . . , 100 to 200 



(e.) Lower Quartzite ; purple^ grey, and white quartzite, 



the upper beds penetrated by annelid burrowings, . 500 to 1 000 

 (Great hiatus and unconformity.) 



Cambrian Beds. 



Eed and purple sandstone^ sometimes pebbly, and towards 

 the base passing into a conglomerate, with large pebbles 

 and blocks of quartz, quartzite, jasper, felstone, gneiss, 

 &c., in thickness from to 3,000 



* Quart. Journ. Geol. Soc, London, vols. xv. and xvii., 1858-61. 

 t " Siluria," 4 Edit., p. 163, witli section p. 169. Figures of the fossils are given by 

 Murchison ; they consist of Maclurea, Ophileta, Oncoceras, and Orthoceras. 

 SciEN. Phoc, K.D.S, Vol. hi., Pt. u. D 2 



