THE RUM VOLCANO AND ITS LAVA STREAMS. 319 



to have been formed on the flanks of the great volcano. The highest 

 mountain in Ardnamurchan is one of these. It rises to a height of 

 1759 feet, and is known as Ben Shiant. It consists of a succession 

 of grassy slopes formed of the softer and less compact rocks, which 

 rest upon columnar lava which terminates its slope in a spur. Ben 

 Shiant rises at the junction of the felspathic and basaltic lavas ; its 

 rocks contain glassy felspar and sometimes include porphyritic 

 pitchstones and rocks like compact felstones. They are referred to 

 the augite andesites by Professor Judd. 



The Island of Rum is a third volcano. Around it lie the smaller 

 islands of Canna, Eigg, and Muck, which are portions of lava sheets 

 more or less interstratified with sedimentary deposits. They accumu- 

 lated gradually, and helped to mark the distance from which the lavas 

 from Rum extended. The foundation of the island of Rum is a mass 

 of Cambrian sandstone with overlying highly metamorphosed rocks, 

 and far away from the lavas the old strata present their normal 

 characters and are free from intrusive dykes. A number of peaks 

 which consist of gabbro rise to a height of about 2500 feet, piercing 

 through the stratified formations. To the east of the gabbro heights 

 is the older mountain of Oreval, which does not rise so high and is a 

 core of granite. Dykes and veins of gabbro almost innumerable 

 penetrate the felsites and granites of Rum ; and, therefore, show that 

 the basic lavas were thrown out subsequently. The ashes ejected 

 from the Rum volcano have nearly all been swept away by processes 

 of denudation, though some patches of felsitic ash still remain 

 preserved by coverings of felstone. 



The Island of Eigg has been described in detail by Professor 

 Archibald Geikie, F.R.S. It is about five miles long, from one and a 

 half to three and a half miles broad, and attains a height of 1300 

 feet. It is an isolated part of a great basaltic plateau, and is so 

 tilted that while it is a thousand feet high at the north, it slopes 

 gently to the south. The volcanic rocks rest unconformably upon 

 estuarine and fresh- water shales and limestones of Jurassic age, which 

 have some marine beds at the top similar to the rocks seen in 

 Raasay. The volcanic rocks covering those beds are a succession of 

 dolerites and tuffs, which vary in thickness from a few feet to fifty 

 or sixty feet, and have an aggregate thickness of eleven hundred feet. 

 They vary in character, being sometimes amorphous and amygdaloidal, 

 and sometimes characterised by columnar structure, which may be 

 radiating. 



Where the rock is amygdaloidal it is largely infiltrated with 

 quartz and chalcedony, calc spar and stilbite. There is a comparative 

 absence of tuffs and agglomerate in the Tertiary period, as compared 

 with their abundance in the Primary era ; but in Eigg many 

 examples occur of breccias interstratified with the dolerite, though 

 they are never thick. As is usual in volcanic districts, intrusive 

 bosses, sheets, dykes, and veins intersect both the underlying oolitic 

 strata and the volcanic rocks. One of these Professor Geikie 

 describes as a quartz felsite, forming a cliff two hundred feet high at 



