I. Lewisian. 



22 C. R. VAN HISE THE PROBLEM OE THE PRE-CAMBRIAN 



Great Britain on that region has just appeared. The succession given 

 for the old rocks of the western Highlands is as follows :^° 



(3. Dolomites and limestones with certain fossiliferous zones. 

 2. Serpulite grit and fucoid beds yielding the Olenellus fauna. 

 1. Quartzites with worm-casts in the upper portions and false- 

 bedded grits below. 



( Unconformability — Plane of marine denudation.) 



f 3. Sandstones and dark micaceous shales. 

 ' 2. Thick series of coarse sandstones and grits with conglom- 

 II. Torridonian . . ^ erate bands. 



1. Dark and gray shales with calcareous bands, fine-grained 

 sandstones and grits with epidotic grits at the base. 



(Strong unconformability, highly eroded land surface. ) 



2. A great series of igneous rocks intrusive in that complex in 

 the form of dikes and sills. 



1. A fundamental complex, composed — 



(«) mainly of gneisses that have affinities, both chemic- 

 ally and mineralogically, with plutonic igneous 

 products ; and 



(h) partly of crystalline schists, which may be regarded 

 as probably of sedimentary origin. 



As to the importance of the unconformity between the Torridonian 

 and the Lewisian, Home says :^^ 



"One of the most impressive features in the history of the Lewisian gneiss 

 is the abundant evidence of prolonged demidation between the cessation of the 

 terrestrial movements just described and the deposition of the Torridon sand- 

 stone. During the protracted interval represented by this denudation the 

 gneiss plateau formed a land surface which was carved into lofty hills with 

 craggy slopes and deep valleys. This fragment of primeval Europe has been 

 preserved under the pile of coarse Torridonian grits and sandstones which is 

 now undergoing slow removal by the agents of waste. The observer may 

 climb one of these Archean hills, following the boundary line between the 

 Lewisian rocks and the younger formation, and note, step by step, how the 

 subangular fragments of horneblende-schist that fell from the pre-Torridonian 

 crags are intercalated in the grits and sandstones, thus indicating the slow 

 submergence of the old land surface beneath the waters of Torridonian time. 

 Between lake Maree and loch Broom it is possible to determine the orientation 

 of these buried valleys and to prove that some of the hills exceeded 2,000 feet 

 in height." 



In the memoir the Lewisian system is often referred to as the "Funda- 

 mental Complex" in exactly the same sense as we have used the term in 

 the Lake Superior region for the pre-Algonkian group. Indeed, the 

 Scotland Fundamental Complex has all the characteristics of that of the 

 Lake Superior region. After years of work by several men, much the 

 larger part of the Lewisian is mapped as "undifferentiated" — that is, it 

 has not been possible in western Scotland, with the splendid exposures of 



10 Memoirs of the Geological Survey of Great Britain. The geological structure of 

 the northwest Highlands of Scotland, 1907, pp. 9-10, 33. 

 " Ibid., p. 4. 



