202 GENERAL REMARKS [Ch. X. 



district throughout the world exhibits some peculiarity in the 

 composition of its primary slates : so that it is possible that 

 the general notion on this subject may be correct, but it is 

 probable that felspar is more commonly present than is sup- 

 posed, from the circumstance of its being distinctly developed 

 in the mica-slates near gneiss; at a greater distance there- 

 from, it may be obscured, as often happens in other rocks, by 

 its intimate union with their ingredients. 



The composition of the primary slates of Cornwall, com- 

 monly called clay- slate or argillaceous schist, has been de- 

 tailed at such length in the fourth chapter, and in such a 

 connected point of view, that it is needless to offer any repe- 

 tition : it may, however, be again remarked, that the two 

 series which immediately succeed the granite, in the eastern 

 and western part of the county, appear to have their equiva- 

 lents in other countries ; the latter series rarely, as in some 

 parts of the Erzgebirge ; the former more frequently, as in 

 the southern part of the primary range of Ireland, in the 

 Western Isles of Scotland, and in Norway, where it immedi- 

 ately succeeds or alternates with gneiss and mica-slate. 



It was at one time supposed that all these primary slates 

 succeeded each other in a certain order ; and that either of 

 them, from gneiss to clay-slate, might immediately repose on 

 granite, but that the consecutive order of Germany, as de- 

 scribed by Werner, was never subverted. This notion, how- 

 ever, is now exploded; principally through the labours of 

 Dr. Macculloch, who taught us that these slates are arranged 

 in Scotland in every possible sequence ; and his statement has 

 since been confirmed by observations made in other countries. 

 Indeed, when we consider how few minerals enter into the 

 composition of the primary slates, and the infinite but similar 

 varieties of modes in which these are united, and the perpetual 

 fluctuation or transition of these minerals into each other, it 

 is not surprising that they are associated together in series of 

 endless permutations. In order to illustrate this subject for 

 the sake of the student, let us follow some of these changes, 



