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



quartzites and Lewisian gneiss lying above the Ben More thrust-plane. The marble is 

 found along the north-east shore of Loch Urigill, about a mile to the south of Loch 

 Borolan, and it is therefore probable that the igneous mass extends for some distance 

 southwards from the latter loch. 



Excellent sections are visible along the banks of the Ledmore River from Loch 

 Borolan to the point where it is joined by the Ledbeg tributary. Immediately to the 

 south of the junction of these two streams there are small exposures of those peculiar 

 types of the igneous rocks, which are specially described in this paper, laid bare by the 

 denudation of the adjacent quartzites and gneiss overlying the Ben More thrust-plane. 



2. Summarising the foregoing evidence regarding the physical relations of the 

 great Loch Borolan igneous mass, it is evident (1) that a zone of crystalline marble can 

 be traced for long distances in immediate contact with or close to the eruptive rock, (2) 

 that a gradual passage can be followed at certain localities from the marble into 

 recognisable bands of the Durness limestone, (3) that on the slopes of Sgonnan M6r and 

 again at Cnoc-na-Glas-Choille the intrusive mass is truncated by the main outcrop of 

 the Ben More thrust-plane, (4) that in the neighbourhood of Ledmore, Ledbeg and Loyne, 

 outliers of the materials overlying the Ben More thrust-plane cover portions of the 

 intrusive rock and the altered Cambrian strata, (5) that from the apparent superposition 

 of the marble along the eastern limit of the igneous mass, it is not improbable that the 

 latter may resemble the other intrusive sills in Assynt, and may have been originally 

 injected as a great sheet along the bedding planes. 



3. Throughout this extensive area, there are striking lithological differences in the 

 character of the eruptive rocks. The prevalent type in the western portion of the mass 

 along the ridge extending from Cnoc-na-Sr5ine eastwards to Lochan Sgeirach is a coarse 

 granitic rock consisting mainly of orthoclase with a little quartz. Occasionally large 

 porphyritic crystals of orthoclase are developed and mica is sometimes present. 



4. But immediately to the east of Loch Borolan, and about three-quarters of a mile 

 to the east of Aultnacallagach Inn, the rock assumes a different phase. Dark brown and 

 black garnets are associated with the orthoclase and a peculiar blue mineral to be 

 referred to presently. The rock is massive, of a greyish or pink tint, unfoliated, and 

 effervesces freely with acid. This type is well developed on the rocky knolls to the 

 north of the road on the slope named Am Meallan on the 6-inch Ordnance Map. 



Not far to the east of this locality there is a small stream (Aultivullin) draining 

 Loch-a-Mheallain and flowing southwards into the Allt Lon Dubh, situated about a mile 

 and a half to the east of Aultnacallagach Inn. Another striking variety is exposed in 

 this burn section above the waterfall. This type is distinctly foliated, with white knots 

 and abundant black garnets set in a dark grey matrix. The dip of the foliation planes 

 is towards the east at an angle of 15°. On the hill slope to the east of this stream and 

 Loch-a-Mheallain this foliated variety of borolanite is conspicuously developed, but the 

 foliation disappears as we pass eastwards towards the limit of the mass. Following the 

 high road from Aultivullin for about half a mile to the east of that locality, the unfoliated 



