12 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



highly metamorphic regions, where the syenite at present exposed 

 would seem inadequate to explain the amount of alteration which 

 the lias-limestones have undergone. 



The syenite of Strath assumes two distinct and easily recognizahle 

 forms ; — one overlying the liassic strata without markedly disrupting 

 them ; the other violently disrupting, but not overlying them. In 

 the former case the junction of the two rocks is horizontal, or rather 

 parallel with the plane of stratification, and the igneous rock con- 

 sequently conforms to the dip and strike of the limestones and shales 

 on which it rests. In the latter case the junction is vertical or nearly 

 so, and the igneous rock breaks through the beds without reference 

 either to their inclination or direction. The one class of phenomena 

 recalls the circumstances attendant on the eruptions of the trap- 

 family, the other reminds one of the appearances that characterize 

 the protrusions of the granites. 



To the latter class belongs that great expanse of syenite of which 

 Beinn na Cailleaich, Beinn Dhearg, and Beinn na'a Cro are the 

 terminal heights on the south, and which, stretching north for eight 

 or ten miles, occupies a large area in the centre of Skye. The only 

 part of this extended district that I have examined is the southern 

 edge abutting against the lias from the head of Loch Slapin to the 

 Sound of Scalpa ; hence among the glens and hill-sides there may be 

 isolated patches of the liassic limestones and shales, although the 

 whole area is coloured and described by Macculloch as syenite. 



The rock of which Beinn na Cailleaich and Beinn Dhearg are com- 

 posed is a granular admixture of brownish felspar and grey quartz with 

 a little hornblende. Occasionally a few scales of mica are observable, 

 so that the rock is fully entitled to rank among the granites. The 

 felspar readily crumbles away, giving rise to long tracks of debris — 

 whence the brownish tint and rounded outline of the hills. 



The line of junction of the lias-beds and syenite can be accurately 

 traced for some distance along the western flanks of Beinn Dhearg. 

 In the bed of the stream which flows down the glen between that 

 hill and Beinn na'a Cro, there occur the limestones and shales already 

 described, dipping away from the hills at angles (decreasing as they 

 retire) of from 50° and 60° to 20°. They cannot be seen in contact 

 with the syenite, but that rock can be traced down to a few yards 

 from them, well displayed in the channel of the stream. From this 

 spot the line of demarcation strikes up the slope of Beinn Dhearg, 

 and, though the exact point of junction always eluded me, the two 

 rocks, judging from the contour of the ground and from the section 

 cut by a stream between Beinn Dhearg Mor and Beinn Dhearg Beag, 

 must have a very nearly vertical line of separation. If there be 

 any inclination, it is probably that of the lias-beds dipping away 

 from and resting against the syenite, which along some parts of 

 the line courses the hill-sides like a ruined wall, its base thickly 

 strewn with prostrate blocks that obscure the line of contact. Oc- 

 casionally, as on the flanks of Beinn na Cailleaich above the milestone 

 on the Sligachan Road, there are large masses of limestone caught 

 up by and resting on the syenite. They do not usually appear im- 



