GEOLOGICAL FORMATION 55 



differences enter into the formation of individual islands 

 which impart to each its distinctive characteristics in 

 landscape and scenery; the composition of the flora also 

 varies to some extent. Few of the islands of the Inner 

 Hebrides are, in detai], identical in structure ; but the Outer 

 Hebrides present us with more of a sameness in formation, 

 their entire length, a stretch of 130 miles, being mainly 

 composed of Old or Lewisian gneiss, the most ancient rock 

 in Britain. Coll, Tiree, and the greater part of lona are 

 similarly formed. In some of the Outer Hebrides, where 

 the vegetation is too scanty to obscure its naked surface, this 

 rock imparts a barren and desolate aspect to the landscape. 

 It attains its greatest elevation in the island of Harris, where 

 it rises to a height of 2662 feet. 



The basaltic islands, from Skye southward to Mull, are 

 of much more recent origin, and consist of consolidated lava- 

 flows erupted during the Tertiary period. Ulster, Mull, 

 Rum, Skye, St Kilda, the Faroes, and Iceland are believed 

 to have been the principal centres of volcanic activity, from 

 which, it is claimed by some, cones arose to a height of 

 15,000 feet, ejecting discharges which overran an area of 

 40,000 square miles. Others hold that the lavas issued 

 more often from fissures than from definite craters, and 

 built up undulating plateaux rather than cones. The 

 numerous north-west basaltic "dykes" of the "Western 

 Highlands furnish ample evidence of the existence of 

 volcanic fissures of this period, although it remains an open 

 question whether these were the chief sources of the lava 

 streams. There is no lack of evidence to show that these 

 islands were once united in one great plateau. 1 



The northern and larger portion of the Isle of Skye is 



mainly composed of Tertiary volcanic rocks. The Cullins 



originated from bosses of gabbro which pierced through 



underlying basalt plateaux; and the Red Hills between 



1 See Appendix. 



