204 GENERAL REMARKS [Ch. X. 



the micaceous gneiss may pass into talc, or the hornblende of 

 the hornblendic gneiss into diallage ; each, perhaps, a mag- 

 nesian variety of the mineral from which they have proceeded 

 by transition : and, in either case, similarly constituted com- 

 pounds as those just described, will be produced, as seen in 

 the magnesian rocks associated with micaceous gneiss in 

 Norway, or more extensively developed in the Alps, which 

 have been well described by Brochant, and quoted at length 

 in the fifth chapter. 



Each of these distinct series of gneiss is capable of passing 

 into various kinds of clay-slate, by the intimate combination 

 of their constituents, into a homogeneous schist, when the 

 characteristic mineral is no longer perceptible, in consequence 

 either of its minute proportion or gradual and total absence. 



Since, therefore, we find that all the primary schists do, by 

 changes in their composition, and in the modes of mineral 

 aggregation, mutually pass into each other, it is not surprising, 

 as above observed, that these transitions should be confined 

 to no determinate order : thus, mica-slate may succeed 

 granite, and yet in its succeeding beds become gneiss by con- 

 taining felspar; so also hornblende-schist or clay-slate may 

 be similarly circumstanced, and yet graduate into mica-slate or 

 gneiss ; and, subsequently, any or all of these may alternate 

 together in different series. 



In examining, however, every series of the primary slates, 

 some portions of each will be found to be very compact, 

 exhibiting no trace of the schistose structure. These compact 

 varieties are sometimes disposed in regular beds, alternating 

 with the perfect slate ; at other times, they occur as masses 

 more or less irregular, completely enveloped in the slate, 

 terminating here and there in protuberances or veins : all of 

 these masses are closely connected with the containing slate, 

 by gradually passing into each other ; and sometimes, indeed, 

 but less frequently, the massive variety encloses irregular 

 portions of the schistose. 



These two principal kinds of the schistose group possess 



