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in Sutherland and Lewis, southward to the islands of Islay and Jura, 
and by a close examination of the stratigraphical relations, is now 
prepared to furnish proof of the truth of the conclusion to which by 
a species of instinct he had been led before. In one of the facts 
upon which this determination rests, I think I can venture, from 
personal observation, to confirm his argument. The term gneiss 
had been correctly applied by M‘Culloch to the fundamental rock 
of the outer Hebrides, a rock which reappears in great mass on the 
south-west coast of Sutherland. But unfortunately he applied the 
same term to other rocks, which are now proved to overlie beds 
containing Lower Silurian fossils. He thus confounded strata which 
are separated by immense ages from each other. Now, Sir Roderick 
Murchison has pointed out the essential differences of lithological 
character which distinguish the fundamental gneiss from all the 
rocks of the overlying series. When these differences are once 
pointed out. it is impossible to mistake the two. The fundamental 
gneiss is distinguished by the predominance of hornblende, so 
thickly laid, generally in lines parallel to the stratification, as fre- 
quently to render the stone almost black. The felspar and mica are 
generally found in large separate crystals and plates ; and it is not 
unfrequently intersected by veins and masses in which the same 
mineral constituents are more perfectly mixed in the form of granite. 
To this rock, which is largely developed in our North American 
possessions, where also it is succeeded by a very similar series of 
overlying deposits, the term “ Lawrentian” has been applied by Sir 
William Logan. 
This term Sir Roderick Murchison proposes to retain for the 
oldest stratified rock yet known in the world. Upon this funda- 
mental Lawrentian gneiss are piled the vast series of Cambrian 
strata which constitute the great mass, and sometimes the whole, of 
the most striking mountain-forms on the west coast of Sutherland and 
Ross. These strata are estimated by Murchison to measure some 
ten or twelve thousand feet in thickness. Resting again unconform- 
ably upon these Cambrian beds, and capping with their white 
quartzites many of the mountains, the true Silurian rocks appear, dis- 
tinguished — -mainly in the limestone bands, but also, though more 
rarely, in the quartzites — -by orthoceratites, and other characteristic 
fossils. Intercalated among these, and therefore having their rela- 
tive age clearly determined, occur those other more crystalline and 
