Ch. X.] ON THE PRIMARY ROCKS.. 197 



dimensions ; all of which, occasionally, send out ramifications 

 into the slate. These appearances are common in Cornwall, 

 and have been frequently noticed in the crystalline slates of 

 other countries, more particularly in gneiss and mica-slate ; 

 when they have been, from theoretical considerations, some- 

 times denominated granitic gneiss ; at others, granite-veins. 

 Let their nature, however, be what it may, there can be little 

 doubt that all these phenomena are analogous to the detached 

 granitic rocks in the slates of Cornwall. 



It has also been shown, in the preceding pages, that the 

 junction of the principal masses of granite and slate exhibits 

 precisely the same appearances as those displayed by the 

 union of the small insulated portions of granite with the schists 

 in which they are contained: viz. they are often gradually 

 assimilated to each other in composition ; they not only 

 mutually enter into the formation of the same layer or bed, 

 but generally form integrant parts of the same concretion, 

 in which case they are not disunited by the lines of structure, 

 and are sometimes traversed by porphyritic crystals of felspar, 

 as at Polmear Cove, on the north coast of Cornwall; they 

 both contain angular and rounded portions of each other ; 

 and, lastly, the granite traverses the slate in the form of 

 veins, which either run their course through both rocks, or 

 proceed immediately from the granite, as elongations of 

 the latter, with which they coalesce at the line of junction, 

 or at no great distance therefrom within the mass of granite ; 

 and these veins, in their modes of connection with the slate, 

 present a repetition of the same phenomena as at the junction 

 of the main masses, even the granite of the vein and the 

 schist partly containing the same porphyritic crystals, as at 

 Bunawe (see chap, viii.) ; where the hornblende-crystals of 

 the granite-veins "may be observed shooting far into the 

 body of the schist, so as to render it often difficult to assign 

 the limits of each rock ; and in a less degree, the quartz and 

 felspar exhibit the same appearance." 



Thus we find, that the granite occurs in immense irregu- 



o a 



