H 1>M . G K O L O G V . 4,9 3 



forms the summits of some nameless hills which lie to 

 the north and east of Baikeval, where it reposes on the 

 augit rock ; and it also constitutes the chief parts of 

 Baikeval, Arsteval, Tralival, and Rishinish, if not the 

 whole of some of these mountains. Its superiority of 

 position may be traced at the foot of these hills in different 

 places. In extent, as well as disposition, it differs entirely 

 from the trap just described, since it forms independent 

 mountains. Portions are also found scattered here and 

 there among that trap and the augit rock ; and in these 

 situations the contrast between the external aspect of the 

 two is very striking. The variety first described is in- 

 variably covered with grey lichens, and breaks into the 

 most common forms assumed by those traps which are 

 not columnar. But the rock in question has every where 

 the aspect of rusty iron, no lichen or vegetable matter 

 attaching itself to the surface ; and by this character it 

 can be distinguished at a great distance. It gives a 

 singularly barren and desolate look to that part of Rum 

 which it occupies ; and in this respect, as well ;'as in its 

 russet hue, strongly reminds the spectator of the naked 

 and sterile appearance of Loch Scavig. The natural frac- 

 ture of this rock is also quite different from that of the 

 former, and is indeed exceedingly remarkable : it is best 

 observed in the smaller hills. The general mass appears 

 in these to consist of an aggregation of huge protuberant 

 bodies, each having its own curved surface. This surface 

 is cracked throughout into large polygonal forms, varying 

 in the numbers of their angles. The fissures which sepa- 

 rate these polygons are three or four inches in breadth 

 on the surface, but approximate as they descend deeper. 

 When therefore a vertical section of these hills is exa- 

 mined, the polygonal masses will be found in contact 

 at a certain depth, the intervals always widening as they 

 ascend. The polygons are easily detached at the surface, 

 and appear in the form of flat slabs, such as would be 



