1 6 Jourjial of Travel and Nat lira/ History 



mountains of red Cambrian sandstone as well as from the surround- 

 ing slopes of volcanic rock. Not only so, but large pieces of 

 coniferous trees were likewise swept down and buried in the gravel 

 of the river-course. At last, however, the volcanic forces, which 

 seem to have been quiescent at this locality for a long period, 

 broke out anew. Successive streams of a black glassy lava, called 

 pitchstone-porphyry, were erupted. These found their way into 

 the river-bed and flowed along it, solidifying, of course, as they 

 went, until the old river-channel was filled up and buried under at 

 least three or four hundred feet of solid rock. Then the work of 

 denudation went on as before. The sides of the old valley, formed 

 of the earlier series of volcanic rocks, continued to be worn away, 

 but the very compact flinty pitchstone, though it too had to yield 

 to the attacks of time, did so at a much slower rate. Hence, by 

 degrees, the former hills and valleys crumbled away, while the 

 pitchstone that had solidified, like so much cast iron, in the bed 

 of the antient river, retained its place. But it no longer winds 

 along a valley. The higher grounds and slopes that once bounded 

 it have disappeared, and it now runs as a huge wall of rock along 

 the crest of the highest ridge of the island. And, at the foot of 

 the precipice, you may dig out from underneatli this great mass of 

 once melted rock the gravel and drift-wood which had gathered in 

 the old water-course before it was sealed up under the river of lava. 

 Here again we see how by denudation hills have been worn down 

 into slopes and valleys, while valleys have been changed into hills. 

 Not less marvellous are the transformations whicli the same pro- 

 cess of erosion has eftected among the other volcanic islands of the 

 inner Hebrides. In Mull, for example, the nearly horizontal sheets 

 of dolerite, basalt, and other hard crystalline varieties of old lavas, 

 rise above each other, terrace over terrace, to a height of some- 

 times more than 3000 feet above the sea. Yet these extremely 

 tough and compact masses have been scooped out into wide, long, 

 and deep valleys, now occupied in part by the sea as sea-lochs or 

 fjords (fig. 6). The edges of the horizontal beds on one side of 

 a valley can readily be connected by the eye with their continua- 

 tion on the other side, and the observer sees before him a proof 

 that these valleys, though sometimes several miles broad, and 

 measured from the crest of the ridges on either side, 1500 or 2000 

 feet deep, can have been produced by no mere convulsion or fracture, 

 but must have been dug out by the powers of denudation acting 

 on the surface. And he learns, moreover, that the mass of rock 



