Ch. V.] PRIMARY SCHISTOSE ROCKS. 67 



3. Granite, with a few thin layers of slate 18 feet wide. 



4. Clay-slate - ,-18 



5. Granite - - .,,,*>* 6 



" About one quarter of a mile from the meeting of the two 

 Avons, there is a fourth and a fifth bed of granite, similarly 

 composed ; each about nine feet thick, separated by clay-slate 

 of thirty feet in thickness." 



We cannot conclude this summary of Mr. Weaver's 

 valuable observations on the primary rocks of the East of 

 Ireland, without recommending to the student a careful 

 perusal of the original. 



Dr. M'Culloch, in his excellent work on the Western 

 Islands of Scotland, has furnished us with copious details 

 concerning the primary schistose rocks. We will, therefore, 

 avail ourselves of this vast fund of information, to select such 

 descriptions as our limits will admit of; more particularly 

 concerning the gneiss, and the various rocks with which it is 

 associated. 



The mineral composition of the gneiss of these islands is 

 the same as that of this rock on the mainland, viz. felspar, 

 quartz, and mica ; but the latter mineral is more frequently 

 replaced by hornblende. The varieties of gneiss, as in the 

 case of other primary rocks, depend not only on the propor- 

 tion in which its respective minerals are united, but also on 

 the mechanical manner in which these are arranged. Founded 

 on the latter circumstance, the true and well-characterised 

 gneiss may be divided into two kinds, the granitic and schis- 

 tose ; both of which, however, will be found frequently to 

 pass into each other by almost imperceptible gradations : this 

 arrangement, therefore, is only one of convenience. 



These kinds are of such frequent occurrence, that a single 

 example of each must suffice. In South Uist*, the gneiss is 

 almost uniformly of a granitic aspect ; and its external forms 



* Western Islands of Scotland, vol. i. p. 95. 

 F 2 



