BARRA. GEOLOGY. 83 



soil, but appearing to have once contained such veins, 

 of which the exposed portions have been washed out. 

 But those which render the gneiss of Barra remarkable 

 are of very small size, and distributed in a manner of 

 which no corresponding examples have occurred to me 

 in any part of Scotland but the Long Island. They 

 are subdivided into branches of extraordinary tenuity, 

 traversing and reticulating the gneiss or the granite veins 

 in the most intricate manner, as either of these happens to 

 lie in their way. However small these veins, even though 

 reduced to the diameter of a thread, they maintain their 

 distinctness from the including rock, never diffusing them- 

 selves throughout its substance, or entering into any com- 

 pound with it, but always remaining defined by a deter- 

 mined boundary. They are readily distinguishable on a 

 fresh fracture ; and when the rock is exposed to the 

 weather, it is equally easy to recognise them by their 

 rusting and falling out, while the gneiss retains its re- 

 fractory nature and aspect. In some places they are so 

 abundant as to equal or exceed in quantity the rock 

 which they traverse. Of this there is a remarkable 

 example at Cuire. The original rock is a dark gneiss 

 containing much hornblende and intersected by veins 

 of grey granite. The trap veins are so numerous as to 

 have separated the rock into small irregular fragments, 

 so that the whole has at first view the aspect of a con- 

 glomerate consisting of fragments of gneiss and granite 

 imbedded in a basaltic paste. Where it has been exposed 

 to the weather, these have been so unequally acted on 

 that it puts on the appearance of a tufaceous lava. I 

 must add, that the matter of all these veins, whether 

 great or small, is invariably identical, and is a very com- 

 pact fine-grained black basalt. No large vein of basalt 

 is to be observed in the neighbourhood of Cuire. 



