GEOLOGY OF SCOTLAND. 23 



that of the formation termed by Werner the 

 Transition rocks, but to which the title of Silurian 

 system is now applied. It consists in a great 

 measure of Grauwacke, limestone, and slate rocks, 

 and in some localities the plutonic or unstratified 

 rocks make their appearance, such as granite, 

 porphyry, and basalt. The Central region, of which 

 the boundaries have been already stated, consists, 

 like the preceding, of formations belonging to the 

 Palaeozoic epoch, and chiefly of the Devonian and 

 the Carboniferous formations; the former con- 

 sisting of the old red sandstone, conglomerates, 

 and other strata of an analogous character ; and 

 the latter of layers of ironstone, limestone, and 

 beds of coal. The third, or most Northerly region, 

 comprehending within its limits a space nearly 

 equal to both the divisions already mentioned, 

 is composed for the most part of the metamorphic 

 rocks, known as the Gneiss system, and contains 

 in addition a large development of the old red 

 sandstone strata, as well as numerous instances of 

 the amorphous crystalline rocks, known as the 

 Granitic system, and comprising granite, porphyry, 

 and trap. 



A striking dissimilarity prevails between the 

 geology of England and Scotland, not only on ac- 

 count of the large development of the unstratified 

 and plutonic rocks in the latter country, but the 

 almost total absence of the more recent tertiary 

 formations. According to the estimates of geolo- 

 gists, the trap rocks of Scotland occupy about one 

 seventh of the area of the country, whereas in 



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