B. K. N. Wyllie & A. Scott— Plutonics of Garahal Hill. 499 



IV. — The Pltjxonic Eocks of Gaeabal Hill. 



By B. K. N. Wyllie, M.A., B.Sc, and Alexandek Scott, M.A., B.Sc, 

 Carnegie Kesearch Scholars in the University of Glasgow. 



THIS complex of igneous rocks lies between Ardliii, at the head of 

 Loch Lomond, and the head of Loch Fyne. In 1892 it was the 

 subject of a fairly exhaustive paper by Teall & Dakyns,^ but in view 

 of a number of new facts which we have discovered we venture to 

 submit a re-examination of the problems connected with the complex. 

 The mass is broken by a north-east fault. The country to the west 

 of this fault is mainly porphyritic granite and tonalite ; to the east 

 there is tonalite and also more basic diorite and ultrabasic rocks. 

 We shall concern ourselves mainly with the basic and ultrabasic 

 groups, as they are somewhat unusual in the " I^ewer Plutonic Eocks 

 of the Highlands ". 



Ultkabasic Eocks. 



The ultrabasic rocks form several small isolated outcrops along 

 a line running N.N.E. from Loch Garabal, and passing about 

 200 yards to the east of Lochan Beinn Damhain (Loch Een Davain). 

 The largest of these covers about one-eighth of a square mile, and 

 lies immediately north of Loch Garabal. 



About 200 yards east of the north end of Lochan Beinn Damhain, 

 and near a little lochan, another exposure appears, followed to the 

 north by two others. The total area is about 200 square yards. 

 A mere patch is also found about 100 yards south of the small 

 lochan, and further north still, at a place 400 yards north of the 

 highest point of Garabal Hill, a congeries of weathered blocks 

 probably indicates another minute outcrop. The Geological Survey 

 map of this area claims a long narrow strip, further north than our 

 last-mentioned exposux'e, as peridotite. But of this no evidence was 

 found beyond a few scattered boulders.^ The map in the paper by 

 Teall & Dakyns includes likewise some peninsulas and islands of 

 schist in the plutonic rocks. A careful examination of these localities 

 proved the outcrops to be partly schist and partly diorite, the field 

 relations of the two, however, indicating that the schist represents 

 the residuum of the original 'roof, which has so far escaped total 

 denudation. A similar schist roof has been recorded by Barrow 

 in Glen Tilt.^ It is noteworthy that this chain of exposures is 

 practically coincident with the line which separates the diorites from 

 the later tonalite. But we have not been able to decide whether the 

 ultrabasic masses are actually wedged in between these two or 

 , enveloped in the diorite close to its margin. On the diorite side 



^ " Plutonic Eocks of Garabal Hill and Meall Breac " : Quart. Journ. Geol. 

 Soc, xlviii, pp. 104-21, 1892. 



'^ The sketch-map of Teall & Dakyns is reproduced in Hatch's Petrology 

 (p. 303), but with the omission of all the peridotite areas we have mentioned, 

 except the one we failed to find, while the positions of Garabal Hill and 

 Garabal Cottage are interchanged. 



^ Geology of Upper Strathspey, etc. (Mem. Geol. Surv. Scotland), 1913, 

 p. 72. 



