160 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



into it. On the south-eastern side of the same hill, still more striking evidence is 

 presented of the posteriority of the acid to the basic rocks. The porphyry shows here 

 the same tendency to assume a bedded structure, the parallel " beds " again dipping 

 outward or SE. at 40°. They plunge under the body of gabbro, dolerite, and other 

 intrusive masses which from this point stretch eastward into the great cones of Allival 

 and its neighbours. The rock at the junction is a fine microgranite with traces of micro- 

 pegmatite. It is composed of a holocrystalline base of quartz and orthoclase, with 

 porphyritic crystals of microcline, blebs of quartz and scattered granules of augite. The 

 rocks that rest immediately next it are basalt and dolerite, into which it has sent an 

 intricate network of veins (fig. 52). # It sends also long tongues down the slope into 

 these rocks, some of which may be seen traversing the dolerite and gabbro veins that cut 

 the basalts. The basic rocks next the porphyry have been intensely altered. They seem 

 in places as if they have been shattered by some explosive force, and had then been 

 invaded by the mass that rushed into all the rents thus caused. The nature of the 

 contact metamorphism produced by the acid protrusions is described at p. 167. 



Fig. 52. — Junction of Quartz-Porphyry (Microgranite) and Basic Rocks, south-east side of Orval, Rum. 

 a, basalts and dolerites ; b, dolerite and gabbro veins ; c, quartz-porphyry cutting a and 6. 



c. Skye. — It is in the island of Skye that the granitoid bosses attain their largest 

 dimensions. They cover there a total area of about 25 square miles, and form char- 

 acteristic groups of hills from 2000 to 2500 feet in height. On the south-east side, a 

 group of three conspicuous cones rises from the valley of Strath (Beinn Dearg Mhor 

 Beinn Dearg Bheag, and Beinn na Caillich). A solitary graceful pointed cone (Beinn na 

 Cro) stands between Strathmore and Strathbeg, while to the north-west a continuous 

 chain of connected cones runs from Loch Sligachan up into the heart of the Cuillin Hills. 

 Their conical outline, their smooth declivities, marked with long diverging lines of screes, 

 and their pale reddish or reddish-yellow hue, that deepens after a shower into glowing 

 orange, mark off these hills from all the surrounding eminences, and form in especial a 

 singular contrast to the black, spiry, and rugged contours of the gabbro heights to the west 

 of them. 



Besides this large continuous mass, a number of minor bosses are scattered over the 

 district. Of these the largest forms the ridge of Beinn an Dubhaich, south of Loch 



* In a thin slice cut from a specimen showing the junction, there is a minute vein of the porphyry penetrating the 

 basalt which is much altered, while the porphyry becomes much finer in grain than at a distance from the contact. 



