Vol. 56.J AND OTHER IGNEOUS ROCKS IN ARGYLLSHIRE. 533 



summer (1899) another mass has been mapped in Glen Orchy, and 

 Mr. E. G. Symes has reported yet another occurrence of the rock 

 near Loch Avich. Thus we have, besides the exposure near 

 Ballachulish, one exposure in the Ben Cruachan area, namely, in 

 Glen Orchy ; two exposures in the Loch Avich area ; and two 

 exposures in the neighbourhood of Ben Bhuidhe. It is believed 

 that these six exposures constitute the only known occurrences of 

 this rock in Britain. 1 It will be seen, therefore, that the area over 

 which the rock is distributed is a considerable one, extending from 

 Ballachulish on the north-west to Ben Bhuidhe on the south-east ; 

 and from the upper part of Glen Orchy on the north-east to the 

 Loch Avich area on the south-west. 



In the course of our work on the Geological Survey, we were 

 enabled to revisit the Ben Bhuidhe district in June 1899, and to 

 collect the additional facts with regard to these peculiar intru- 

 sions, and more especially the evidences of their relationships with 

 the diorites and granites of the district, which are presented in the 

 following pages. 



Kentallenite occurs as intrusive masses, varying in size, in the 

 younger Highland schists of Argyllshire. Occasionally these masses, 

 as in the neighbourhood of Loch Avich, have a dyke-like behaviour. 

 More usually, however, the intrusion takes the form of a lenticular 

 or roughly oval-shaped mass. The larger of the two Ben-Bhuidhe 

 intrusions forms a broad oval mass, exposed for a distance of | mile 

 in Brannie Burn (see Map, p. 542). In Glen Orchy again is a more 

 -elongated lenticular intrusion, well exposed in the burn flowing into 

 the river immediately above the Falls of Orchy. In each locality, 

 the rock has a similar aspect, and shows a. characteristic mode of 

 weathering : it is black when freshly fractured, and weathers to 

 a rusty brown. Huge, rugged, and disintegrated blocks mark the 

 outcrop of the intrusion, and show a curious pitting of their surface 

 from the action of the weather. A weathered block or boulder may 

 frequently be seen to be covered with numerous cup-shaped de- 

 pressions (up to 9 or 10 inches in diameter), often resembling the 

 potholes in the bed of a stream. These will hold rain-water and 

 growths of moss end lichen, and so assist the action which doubtless 

 such growths largely contributed in starting. 



At Kentallen the rock has been quarried for many years, and is 

 locally known as ' black granite.' A hand-specimen is frequently 

 coarse in texture, and biotite, augite, and olivine can be easily 

 recognized with the naked eye. The brown mica occurs in large 



1 Since the above was written, our colleague Mr. J. S. Grant-Wilson has 

 informed us of two additional occurrences of kentallenite in the Ballachulish 

 area: one in Glen Buror, about 1^ miles south of Kentallen; and the other 

 ^ mile south-west of Palnatrat, and about 4 miles south-west of Kentallen. 

 The Glen-Duror intrusion is a boss-like mass, but is too decomposed for satis- 

 factory description. The other occurrence is in the form of a dyke, and the 

 rock is exactly similar to that of Kentallen. 



