82 BARRA. GEOLOGY. 



with but little exception it will be found to predominate 

 throughout the whole of the Long Island. 



This rock is here so irregular that the beds can rarely 

 be traced for more than a few yards together in a straight 

 line. They are commonly bent and occasionally much 

 contorted, while their inclinations to the horizon are so 

 irregular and inconstant, that no consistency or order 

 can be perceived among them. Still, their leading 

 position approaches the horizontal one ; unlike those 

 of the southern parts of Coll, which not only are elevated 

 at high angles, but show a considerable rectilinear ten- 

 dency. In composition and structure they vary almost 

 every where, the principal differences being produced 

 by the presence or absence of hornblende, or of mica. 

 The micaceous varieties are however rare, and the com- 

 pounds of felspar and mica only, are seldom found to 

 occupy any great space. They rather form the occa- 

 sional laminos of a fundamental mass containing horn- 

 blende in various proportions ; that substance causing 

 the laminar appearance and fissile tendency of the rock. 

 Occasionally this gneiss passes into common hornblende 

 schist, while in certain situations the laminar tendency so 

 far disappears that it assumes the characters of a common 

 granite. 



Granite veins abound as usual in this gneiss, and they 

 have in Barra a character similar to that which they 

 possess in Coll ; the red colour however is predominant. 



In weathering, these rocks acquire an uniform rough 

 surface with a slight convexity, which, together with 

 the fissures that cross each other at considerable angles, 

 and the occasional exposure of the veins, give to the 

 whole an appearance resembling granite very apt to 

 mislead an inexperienced eye, particularly from a distant 

 point of view. 



Trap veins are found traversing the gneiss, attended 

 with some unusual appearances. In many places there 

 tire large rents filled by loose matter and vegetable 



