i3o ORIGIN OF INNER HEBRIDES. 



thicker perhaps than the height of the loftiest mountains of the world, 

 must have taken place in the region of the English Channel before 

 the granite cliffs of the little island of Sark could have been laid bare. 



Inner Hebrides. Finally, there are, in the Inner Hebrides, islands 

 where the surfaces are portions of lava sheets poured out from volcanoes 

 which themselves formed larger adjacent islands, or existed on the 

 mainland. It is difficult to judge of the condition of the west coast 

 of Scotland in those older or middle Tertiary times, when the lava 

 streams were poured out. But not improbably the country was a 

 tableland, and the lava flowed out under the atmosphere. The 

 basalt of the Isle of Mull extends across into the opposite peninsula 

 of Morvern on the one hand, and on the other side forms Ulva and 

 Staffa ; while the Treshnish Isles probably but imperfectly indicate 

 its western extent. The lavas of Eigg may have been emitted from 

 the volcano of Rum. Any one who examines this ancient British 

 volcanic country, and observes the broad and deep channels which 

 the streams excavate for themselves in it, will be at no loss to dis- 

 cover the means by which, aided by depression in level of the country, 

 the sea severed islands like Ulva and Staffa from the adjacent 

 land. A similar valley widened by the sea formed the Sound of 

 Mull. This is almost demonstrated by the fact that the rivers and 

 streamlets all run into the Sound of Mull at right angles to its 

 length, just as though they were tributaries joining a larger stream. 

 Hence these volcanic islands and islets have originated in denudation, 

 and only differ from the other examples mentioned, in the fact that 

 most of the rocks seen on their surface have been poured out in 

 molten streams from cones of volcanoes, instead of being deposited 

 as strata on the floor of the ocean. 



It is instructive to observe that an elevation of fifty fathoms 

 would remove the whole of the Malay Archipelago from the map, and 

 substitute in its place a south-eastern extension of the Asiatic con- 

 tinent. It is even more instructive to remark the vast depths from 

 which atolls in the Indian and Pacific Oceans rise from the sea-bed, 

 since we thereby obtain an idea of the height of mountains which 

 have sunk beneath the sea, and the probable area of lands which 

 have been removed by the depression of islands, since the main out- 

 lines of land and water have been what they are now. 



