Volcanic Rocks. 355 



It is clear that the beds of lignite in the Western 

 Isles, and the shales with leaves, indicate long pauses 

 here and there in the activity of many craters. Vege- 

 tation on a large scale had time to flourish* After an 

 unknown lapse of time, the vast inclined plateaux of lava, 

 above which the lofty craters rose, are still, in Antrim, 

 from 600 to 900 feet thick, and more than 3,000 feet in 

 Mull. The denuded edges of the several lava-streams 

 now form a wonderful series of terraces, rising tier upon 

 tier, like Titanic steps, high on the hills on both sides of 

 the Sound of Mull, and the splendid columnar basalt of 

 Fingal's Cave, in Staffa, is known to all tourists among 

 the Inner Hebrides. The same terraced forms are 

 prominent in Skye, and in many of the smaller islands. 

 But where are the craters from which these vast volcanic 

 piles of lavas and ashes were ejected? They are all 

 gone and utterly wasted away, and only their deep- 

 seated roots remain to mark the sites, above which 

 mountains grew by accretion, as high as Etna, which is 

 growing even now. It is a remarkable circumstance, 

 and worthy to be noted, that these deep-seated centres 

 of crystalline rocks are now apt to form some of the 

 highest portions of the islands. They have been bared 

 by denudation, and their hardness helps to preserve 

 them. 



Long before these extreme denudations took place, 

 when the islands formed part of a wide-spreading 

 territory, old river-beds intersected it, running through 

 an ancient land, formed of Laurentian, Cambrian, and 

 Silurian rocks, that spread far to the west, north, and 

 east. 



These rivers scooped out valleys in the Miocene 

 lavas and tufas, which were again partly filled by 

 torrents of basalt and obsidian. In the case of the 



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