PLUTONIC ROCKS OF GAKABAL HILL AND MEALL BREAC. 105 



rock we enter upon the schist of the country which forms Ben 

 Damhain and the district north of the Arnan for several miles. 

 On the west side of Ben Damhain and north-west of Loch Garahal 

 we have a rock precisely like the tonalite of the Arnan. This rock 

 becomes more basic northwards, and on the slope of Ben Damhain 

 there is a narrow band of diorite between the schist and the tonalite. 

 Westward, however, the tonalite passes into granite which finally 

 becomes porphyritic, owing to the presence of large crystals of ortho- 

 clase measuring often two inches across. 



In some places a sharp line can be drawn between the porphyritic 

 and non-porphyritic granite or tonalite, as shown on the map, 

 but in others, as, for example, west and south-west of Loch 

 Garabal, the porphyritic character comes on quite gradually. The 

 tonalite and non-porphyritic granite extend northwards as far as 

 the Allt-na-Lairige, a stream in Argyllshire on a line with that of 

 the Arnan, but whose waters flow in the opposite direction. North 

 of the Allt-na-Lairige the plutonic mass consists entirely of porphy- 

 ritic granite. 



The boundary on the east of Ben Damhain is a well-marked fault. 

 In other places the original intrusive junctions are often admirably 

 exposed ; both granite and tonalite may be seen in contact with 

 the surrounding schists, and the junctions are of the same character 

 in both cases. That on the east of Lochan Ben Damhain may be 

 taken as a type. As the plutonic area is approached veins of 

 tonalite or diorite make their appearance ; these increase in num- 

 ber and interlace with each other to such an extent that masses of 

 schist become isolated in the plutonic rock. A farther advance in 

 the same direction across the boundary shows that the included 

 fragments gradually disappear. 



As the map indicates, the area occupied by granite is largely in 

 excess of that occupied by diorite. Taking the whole plutonic area 

 as amounting to 12 or 13 square miles (say 12|), 10 of these are 

 occupied by granite and only 2| by diorite. Diorite, therefore, forms 

 only i of the total area. The area occupied by ultra-basic rock is 

 exceedingly small, amounting at the most to J square mile. 



A consideration of the distribution of the different types of rock 

 within the area brings out the fact that diorite is found only near 

 the margin, and especially along the south-eastern margin. "We have 

 never seen it surrounded by granite except as small inclusions. It 

 is especially worthy of note that south and east of the main plutonic 

 mass there are several exposures of diorite entirely surrounded by 

 schist and unaccompanied by granite. Except along the fault, the 

 schist of the country is penetrated by veins of granite or diorite, as 

 we have already stated. 



Now, the greater part of Ben Damhain and the southern slope of 

 Troisgeach are occupied by a peculiar type of schist. The special 

 character of this schist is the occurrence of patches of felspar fre- 

 quently about the size of peas. These often weather out as roundish 

 lumps and give the rock somewhat the appearance of a pebbly grit. 

 That the felspars are not really pebbles is proved by their irregular 



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