104 MESSES. J. K. DAKYNS AND J. J. H. TEALL ON THE 



10. On the Plutonic Rocks of Gaeabal Hill and Meall Beeac. 

 By J. E. Daktns, Esq., M.A., and J. J. H. Teall, Esq., M.A., 

 F.E.S., E.G.S. (Eead January 27th, 1892.) 



[Communicated by permission of the Director-General of the 

 Geological Survey.] 



The schists of the Southern Highlands of Scotland have been 

 traversed in many places by igneous rocks. Sills and dykes are 

 widely distributed throughout the district, and here and there, as 

 for example in Glen Tilt, Glen Lednock, and near the head of 

 Loch Lomond, extensive areas are composed of plutonic rocks. Our 

 object in this communication is to describe some of the phenomena 

 which may be observed in the last-mentioned locality. 



The plutonic rocks in question form the belt of high ground 

 stretching in a south-west direction from Inverarnan. They vary 

 considerably in chemical and mineralogical composition. Diiferent 

 parts of the mass can be proved by the phenomena of veins and 

 inclusions to belong to slightly different periods, but regarded as a 

 whole it must evidently be referred to one geological epoch. When- 

 ever two portions in juxtaposition are seen to differ in age, the more 

 acid, so far as our observations go, is invariably the younger. Thus 

 the veins are more acid and the inclusions more basic than the 

 material surrounding them. 



Eather more than half a mile above Inverarnan tonalite (quartz- 

 diorite) appears in the bed of the Arnan and on each side of the 

 stream. From this point it extends in a south-westerly direction for 

 about 2| miles, as shown on the accompanying map, and possesses 

 a fairly uniform character. Garabal Hill, south-west of Inver- 

 arnan and immediately south of the place where tonalite appears in 

 the Arnan valley, is composed of diorite. In some places, as, for 

 instance, west of Garabal Hill, a sharp line can be drawn between 

 diorite and tonalite, but in other places the transition from one rock 

 to the other is so gradual that it becomes impossible to draw such 

 a line ; consequently parts of the area mapped as diorite between 

 Garabal Hill and Loch Garabal might just as well have been map^Ded 

 as tonalite. On the whole, the south-eastern portion of the plu- 

 tonic belt is more basic than the north-western. 



Loch Garabal is the loch situated nearly a mile south of the 

 summit of Ben Damhain.^ Immediately north-east of this lochan 

 there is a small area of ultra-basic rock (marked as peridotite on the 

 map). Another small patch of similar rock occurs in the diorite north 

 of Garabal Hill. The limits of this rock are fairly well defined. 



Immediately north of the first-mentioned patch of ultra-basic 



^ It has no name on the one-inch map. On the six-inch map it is designated 

 Lochan Strath Dubh-uisge (the lochan of the Blackwater Valley), but it is 

 better known, at least to fishermen, by the name that we have adopted. 



