Ch. X.] ON THE PRIMARY ROCKS. 201 



ferent series of gneiss do not follow each other abruptly, but 

 are gradually connected by beds of the one occurring in the 

 other, at first sparingly, and of a small size, and so increasing 

 in number and dimensions, until one series disappears. 



These general remarks on gneiss are equally applicable to 

 every other member of the schistose group ; the nature of 

 their varieties, of course, depending on the composition of the 

 schist. Mica- slate, according to most observers, consists of 

 mica and quartz united in different proportions, and in diverse 

 modes of aggregation ; in some cases, Macculloch states, that 

 it also contains felspar, especially when it is about to pass 

 into gneiss, with which it is frequently associated. It appears 

 to us very probable, that this combination is of more common 

 occurrence, but that the felspar is not then distinct, being 

 intimately blended with the quartz, giving rise to the ex- 

 tensive series of compounds, commonly called compact felspar, 

 which exhibit every shade of character between perfect 

 felspar and pure quartz ; this opinion is founded on the fact, 

 that all parts of mica-slate districts are not barren ; and in 

 these cases the slate is decomposed to a great depth, so that 

 it is difficult to obtain an unaltered specimen : now mica, when 

 combined alone with quartz, or indeed with felspar in a 

 distinct granular compound like granite, does not appear to 

 be susceptible of decomposition, and therefore could not pro- 

 duce such a soil ; but when all these minerals are intimately 

 blended, they readily disintegrate. On this conjecture, the 

 reason why the binary compounds are better known is, that 

 they are more durable, consequently more exposed to view, 

 as tors and similar projecting masses, and more liable to be 

 resorted to for specimens, on account of their more perfect 

 condition. The same supposition equally applies to shorl, 

 chlorite, and talc-schists ; and it may be remarked, that in 

 Cornwall such combinations do occur, the schists of this 

 country abounding in or being characterised by compact 

 felspar. The analogy is not perfect, nor indeed could any 

 reasoning founded on this fact be conclusive, for each 



