Ch. X.] ON THE PRIMARY ROCKS. 203 



commencing with micaceous gneiss ; that is, a compound of 

 felspar, quartz, and mica. In the first place, according to 

 the size of the constituents and their mode of arrangement, 

 several species of lamellar, schistose, and compact (or granitic) 

 gneiss, will alternate in beds of various dimensions : the gra- 

 nitic gneiss may lose its mica and become pegmatite (graphic 

 granite), or a kind of eurite, according to the size of the 

 grains ; the former being large and crystalline, the latter 

 small and intimately blended or actually combined, passing 

 into granular or massive compact felspar-schists : or the same 

 granitic gneiss may lose its felspar, and graduate into griesen, 

 in which the minerals are distinct; and this again into an 

 uniform compact quartz-rock or avanturine ; and lastly, by 

 the total disappearance of the mica, into a pure quartz-rock : 

 or the granitic gneiss may be deprived of its quartz, when 

 another kind of eurite would result. The more fissile species 

 of gneiss exhibit similar variations, but the species thus pro- 

 duced have not been distinguished with the same minute 

 details, except in one or two instances ; viz. when the felspar 

 is wanting, or when all the component parts, or the felspar 

 and quartz alone, are united into an uniform and homo- 

 geneous texture : the varieties proceeding from the former 

 condition are called mica- slate; from the latter, clay-slate. 

 Such are the leading changes which the simple case of mica- 

 ceous gneiss, devoid of all accessory minerals, may undergo ; 

 and which may be arranged in many different orders, im- 

 parting to a district a great variety of aspects. Let us now 

 suppose the mica, in some parts of the series, to pass into 

 hornblende ; many new compounds would then be produced 

 which may alternate with part or the whole of those just 

 enumerated : thus many species of compact or schistose 

 syenite would be formed by the conjunction of all the 

 minerals, or of compact and schistose greenstones by the loss 

 of the quartz ; or by the homogeneous mixture of the com- 

 ponent parts, traps and basalts, of various appearances, 

 according to the modes of aggregation. Again, the mica of 



