Ch. III.] COMPARED WITH THOSE OF CORNWALL. 29 



seen in the brook which flows down the northern face of the 

 mountain. This rock is not unfrequently porphyritic ; in 

 almost every glen the felspar crystals may be seen, two or 

 three inches in length, inlaid in a small-grained base. But 

 there is another variety of granite, which occurs in many 

 parts of this tract, and requires more particular attention. 

 " The eastern side of Eagle Hill, for example, corresponds 

 with the usual granite of the country in the size of its grain 

 and proportion of its ingredients ; but on its summit, which 

 forms an abrupt face to the north, quartz appears as the prin- 

 cipal constituent, affecting a considerable variety of aspect. 

 On the western side are great masses and blocks of pure white 

 compact quartz, and lower down on that side is the quartz 

 rock itself: on the south, again, the face of the rock is bared, 

 and the granite here appears to be mostly composed of quartz ; 

 the prevalence of which becomes still more striking in the 

 loose blocks, the felspar having decayed, and left a singularly 

 connected tissue of quartz, exhibiting amorphous masses, 

 veins, and ramifications."* 



" In the southern part of this granitic district the granite 

 is not immediately surrounded by mica-slate, but clay-slate; 

 and the granite of the hills, extending from Conna Mountain 

 to Crogan Kinshela, of the eastern bank of the Avonmore, 

 of West Aston, of Kilmancanna Hill, and of the low rocky 

 ridge called Carrigmore, which proceeds eastward, and is 

 connected with the Dunganstown range, possesses characters 

 different from those of the granite of other parts of this tract. 

 Quartz, comparatively speaking, seldom appears in it ; felspar 

 and mica are the prevailing ingredients, sometimes the one 

 and sometimes the other predominating. The felspar is 

 usually of a yellowish or greyish white, unless when coloured 

 greenish by mica ; which mineral approaches sometimes to 

 chlorite on the one hand, or to hornblende on the other. 

 The felspar and mica are sometimes so intimately incor- 



* Geol. Trans., vol. v. p. 132. et seq. 



