RON A. CEOLOGY. 153 



them being penetrated by basaltic veins so as to 

 form a very singular compound rock. One of the beds 

 of gneiss is remarkable on account of its beauty, con- 

 sisting of a fine grained snowy and equable mixture 

 of quartz and felspar in which the several ingredients 

 are scarcely distinguishable. 



The most conspicuous of the trap veins on this coast 

 occurs at the southern end of Rona. It is nearly 

 vertical, being thirty-six feet thick and traversing the 

 schist in a N. N. W. direction. It is rendered interesting 

 by the nests of tufaceous amygdaloid which are irregu- 

 larly dispersed thoughout it and contain nodules of 

 radiated mesotype. 



A great number of islands, some of considerable 

 magnitude are found skirting the northern and western 

 shores of North Uist, and dispersed throughout the 

 sound of Harris.* They seem so unquestionably to be 

 scattered portions of that island, or the fragments 

 of a common mass, that I thought it unnecessary 

 to examine them all, and now think it useless to 

 describe those which I did examine, since they afford 

 room for no remarks in addition to those which 

 have been repeated, perhaps unavoidably, too often 

 already. Their general character may be understood 

 by saying that they consist of rocky bases of greater 

 or less elevation skirted by sandy shores. In general 

 they possess the best pastures on this coast, being little 

 encumbered with peat ; and as they are for the most part 

 low, the quantity of naked rock is much less than is 

 usually observed throughout this country. They are 

 all very productive of sea-weed, from which great part 

 of their revenues are derived. Wherever I examined 

 the rocks I found them to consist of gneiss with no 

 varieties worth recording. 



* For thu names of these the reader may consult the general Map. 



