88 E1USKA, &C. 



Although the gneiss of Eriska presents no features 

 deserving of particular notice, that of Fudia is somewhat 

 distinguished by the magnitude and number of the granite 

 veins which it displays. Lumps of granite, apparently 

 independent of veins, are also seen imbedded in it. From 

 these as well as from the larger veins there proceed 

 branches anastomosing and diverging in a very capricious 

 manner, which the great continuity and extent of the 

 naked surface gives ample opportunities of examining. 

 The felspar of these veins is remarkable for its purple 

 hue, and it is found in large concretions, as is usual in 

 the granite veins which traverse gneiss. Large masses 

 of confusedly crystallized hornblende are also found dis- 

 persed throughout the gneiss. 



A few veins of quartz are seen traversing ., the rock. 

 These are of small dimension, not exceeding two or 

 three inches in breadth, yet they present circumstances 

 of some importance in the history of gneiss. It has been 

 generally supposed that the quartz veins of this rock 

 as well as those of mica slate, were of contemporaneous 

 origin with the substances in which they lie, or, at least, 

 that they have been formed by a secretion of quartz 

 into cavities produced by the shrinking of the adjacent 

 parts during the process of induration. But these veins 

 are here attended by a distinct shifting of the rock which 

 they traverse ; a circumstance sufficient to prove their 

 posteriority, and the forcible dislocation of the parts 

 which bound them, at a period more recent than that at 

 which the rocks acquired their form and disposition. In 

 the instance under review the changes in the gneiss belong 

 to two distinct periods, since it is first displaced by the 

 intrusion of the granite vein, while the gneiss and the vein 

 together are subsequently shifted by the quartz. The 

 subjoined drawing* will serve to give an idea of the 



* PI. II. fig. 3. 



