Ch. VIII.] THE GRANITIC AND SCHISTOSE ROCKS. 145 



That these appearances have not been more commonly 

 observed, may be attributed to the want of favourable sections, 

 and not to the primary rocks of other countries being dif- 

 ferently constituted : an example of this nature has, indeed, 

 been recorded by Mr. Weaver, as occurring in the eastern 

 part of Ireland. " The brow of Tonelagee," he says, " ex- 

 hibits bold precipices from four to five hundred feet in height. 

 The northern and southern portions of this are composed of 

 granite ; but, in the interval, there is a body of mica-slate 

 about two hundred yards wide, including a bed of granite 

 which varies from six to ten yards in width, besides irre- 

 gularly formed masses of the same rock imbedded in and in- 

 corporated with the mica-slate. It appears to be a prolongation 

 of the body of mica- slate at the head of Glenmacanas, 

 gradually narrowed in its western progress, and probably 

 tapering to an edge, so as to constitute a kind of wedge- 

 shaped mass inserted in the body of the granite. " * 



If any doubt should still remain as to the occurrence of 

 granitic rocks as insulated masses in slate, it may be stated, 

 that this does actually take place in the Herland mines, which 

 are more than a mile distant from the nearest body of granite. 

 Indeed, the irregular veins and bunches of granite in the 

 gneiss of Scotland and Norway are of the same nature : they 

 have been sometimes called granitic gneiss, and have been 

 attributed by others to an injection of granite ; but let their 

 origin be what it may, they are evidently analogous to the 

 insulated portions of the granitic rocks in the slate of Corn- 

 wall. Refer to the examples quoted in the fifth chapter, and 

 more particularly to Macculloch's description of the Western 

 Islands of Scotland, where " lumps of granite, apparently 

 independent of veins, are often imbedded in the gneiss ; and 

 when they are large, commonly give out branches or veins, 

 which diverge in a very capricious manner." 



The granitic rocks are thus disseminated throughout the 



* Geol. Trans., vol. v. p. 145. 

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