84 Prof. Forbes (?M the Topography and Geology of the 



excursion from the hospitable mansion of Ru Dunau, on 

 Loch Brittle.* 



We now come to the more special geology of the Cuchulr 

 lin group. 



The mass, as already said, is composed of hypersthene 

 rock, — a compound of the mineral of that name with felspar. 

 Its boundary, which is a point of much interest, is, generally 

 speaking, tolerably defined by the form of the mountain 

 group. There is, however, an important exception on the 

 eastern side, where the boundary of the hypersthene and the 

 felspathose rock, forming the adjoining formation (which be- 

 comes the porphyry and sienite of the Red Hills), is pecu- 

 liarly distinct. The hypersthene boundary crosses the valley 

 of Glen Sligachan, in its upper part, near Loch-na-Nain, 

 over to the foot of Ben Blaven, — and though unable to speak 

 with perfect confidence (having, by an accident, b^en pre- 

 vented from ascending that hill), constitutes apparently the 

 whole of Blaven, and part of the neighbouring hill of Corry- 

 pin, extending I know not exactly how far, but by its in- 

 fluence at least, probably a good deal farther east into the 

 heart of the syenite hills, between Loch Slapin and Loch 

 Eynort. In the neighbourhood of Glen Sligachan, especially 

 near the mouth of Hart-o-Corry, the sharp unequivocal sepa^ 

 ration of the deep green hypersthene, as it overlies the pale 

 orange felspar rock, may be discerned for miles ; and the 

 contact is immediate. Now the most precise expression 

 which can be found in Dr M.'s writings on the relation of 

 the syenite and hypersthene, is, that " although the former 

 is found within a short distance of the latter, the intermedin 



* I have vainly endeavoured to obtain more information on the sub- 

 ject from the Doctor's later volumes of Letters on the Highlands ; a work 

 which made some noise from its personality, but which is nevertheless 

 marked by no common talent. But here we are even more at fault. We 

 are carried blindfold back and forward, right and left, apparently for no 

 other purpose but to mislead us, as to the direction and extent of the 

 author's own peregrinations. The result, however, confirms the impression 

 stated above. 



