MR J. HORNE AND MR J. J. H. TEALL ON BOROLANITE. 167 



2. The detailed mapping of the region has also shown that these igneous intercala- 

 tions are more or less confined to certain definite horizons in the sedimentary strata. 

 Several sheets are interleaved in the Torridon sandstone, which rests unconformably on 

 the eroded platform of Archaean gneiss, while in the overlying Cambrian strata, two 

 occur in the basal quartzites, two in the " Pipe-Eock," one in the " Fucoid Beds," two in 

 the lowest group of limestone, and one in the succeeding group of Eilean Dubh limestone. 

 These intrusive masses are not always traceable, some of the bands being much more con- 

 stant than others, but in the area surrounding Inchnadamff they are typically developed. 



3. It is rather remarkable that this outbreak of volcanic activity in these ancient 

 sedimentary systems is comparativ elylocal, for though the Torridon sandstone and the 

 overlying Cambrian strata can be traced continuously for a distance of 90 miles across the 

 counties of Sutherland and Eoss, the igneous rocks are confined to a limited portion of 

 this belt. In the area lying to the west of the post-Cambrian terrestrial movements, 

 they extend from Loch Assynt to near Elphin — a distance of about nine miles, but in the 

 region affeeted by these movements they stretch from Glencoul to Ullapool — a distance 

 of 24 miles. Originally they must have penetrated far to the east, for they are 

 carried westwards with the associated sedimentary strata along the higher thrust-planes. 



4. From the fact that the intrusive sheets are truncated by the numerous thrusts or 

 lines of displacement traversing the region, it is obvious that the period of volcanic activity 

 to which they belong is later than the Cambrian limestone of Durness and older than 

 the post-Cambrian movements. 



III. Intrusive Mass of Cnoc-na-Srdine, Loch Borolan, and Ruighe Cnoc. 



1. In the southern portion of Assynt, there is a remarkable development of these in- 

 trusive igneous rocks, covering an extensive area from Ledbeg eastwards to a point near 

 the road leading to Loch Ailsh — a distance of 5 miles. They can also be traced from 

 the peat-clad moor south-east of Loch Borolan northwards to the slopes of Sgonnan 

 M5r. The particular group of rocks which are specially described in this paper are 

 associated with this great intrusive mass. # 



The relations of this extensive series of igneous rocks to the surrounding strata are of 

 special interest and in the neighbourhood of Ledbeg and Ledmore are rather complicated. 

 In the latter region there are various outliers of materials lying above the Ben More 

 thrust-plane, originally continuous but now occurring in isolated patches, which cover 

 alike portions of the igneous rocks and the adjacent marble. Notwithstanding these 

 complications, there are several sections defining the limits of the intrusive rocks and 

 their relations to the altered Cambrian limestone. 



Between Ledbeg and the road leading to Loch Ailsh the eruptive rocks form a range 

 of hilly ground rising to a height of 1305 feet in Cnoc-na-Sr5ine. From Loch Borolan to 



* The description of the physical relations of this intrusive mass may be more readily followed by referring to Sheet 

 101 (one-inch) of the Geological Survey Map of Scotland. 



