ER1SKA, &C. 89 



appearances in question, as well as of a still more recent 

 change induced by the intrusion of a trap vein. 



The last circumstance in Fudia worthy of notice, is the 

 existence of oxidulous iron in the granite veins. This is 

 accompanied by black mica, and forms detached masses 

 of the size of an apple, or thereabout, protruding from the 

 surface of the rock. 



Although Hellesa and Gia correspond in every respect 

 with Barra in structure and character, they exhibit one 

 of the most striking features of that island in a still more 

 accessible form. This is the passage of the trap veins 

 through the gneiss. It is difficult, perhaps impossible, in 

 Barra to trace the reticulations of the basalt to a principal 

 mass : a geologist therefore, limiting himself to that island, 

 might easily pass over the whole without remarking this 

 phenomenon, or he might neglect it as one of the ordinary 

 forms of contorted gneiss, or, if he observed it, remain 

 unable to explain it for want of a clue. It is one of those 

 recondite appearances in the disposition of rocks which 

 points out the necessity of rigid and minute research 

 combined with the more general view of broad and leading 

 characters. On the shores of these small islands the larger 

 trap veins are seen in various places, and they are easy of 

 access. They are in general distinctly pointed out by the 

 vacuities which the action of the sea has caused. The 

 water gradually insinuates itself, not only between the walls 

 of the veins and the body of rock, but among the trans- 

 verse rifts of the veins themselves, detaching them in 

 masses which the waves wash away, leaving open rents 

 and semi-cavernous appearances. These veins vary from 

 three to six feet in thickness, and are generally in an 

 upright position, but affect no particular direction. It 

 is not difficult to pursue their ramifications, which pene- 

 trate the adjacent rock, often to a considerable dis- 

 tance, producing the appearances already described in 

 speaking of Barra. It will hereafter be seen that 



