8 2 The Scottish Naturalist. 



probably up to the middle of the Eocene age, Eigg has no geo- 

 logical history that can be read by us to-day. Whether these 

 successive systems were laid down on the ancient plain, on 

 which Eigg stands, we do not know for certain ; but, as among 

 the materials blown out of the neighbouring extinct volcano of 

 Mull, numerous chalk flints are found, it seems probable that 

 Cretaceous rocks had, at one time, been superimposed upon these 

 older Oolitic strata, and had since been mostly removed by 

 denudation. At anyrate in early Tertiary times a plain of Jurassic 

 rocks spread all over the region now occupied by the Inner 

 Hebrides, probably a flat prairie land, as, from the soft and uniform 

 character of these rocks, they are readily plained down by denud- 

 ing agencies. 



At this time the great mountains of Skye and Rum and Mull 



" Ulva dark and Colonsay 



And all the group of islets gay 

 That guard famed Staffa round," 



as well as " famed Staffa " itself had no existence whatever, or 

 existed potentially, only as so much volcanic ammmunition buried 

 deep in the bowels of the earth, while the gneissic mass of Iona 

 probably stood a great isolated hill in the far extending plain. 



Probably about the middle of the Eocene period this plain be- 

 came the scene of one of the most tremendous exhibitions of 

 volcanic phenomena of which there is any geological record. In 

 what are now Skye, Rum, Ardnamurchan and Mull fissures opened 

 in the ground, from which gases, previously confined under 

 enormous pressure, rushed with terrific violence, blowing far into 

 the air a vast quantity of fragments of the lava in which they had 

 been imprisoned, along with portions of the rocks through which 

 they had been erupted ; these fragments, for the most part, rained 

 down round the respective orifices, but were also scattered far and 

 near over the ground, so that specimens of the rocks of all the 

 formations from the Laurentian Gneiss to the Chalk are found 

 among the ruins of these volcanoes. As the eruption continued 

 great piles of pieces of lava and the stratified rocks were formed 

 round the orifices, that is to say great crater cones were formed ; 

 and by-and-bye the lava welled up, and bursting forth either through 

 the loose material of the craters or the rocks in their neighbour- 

 hood, poured out over the plain. 



The earliest lava streams that flowed from the Hebridean 

 volcanoes were of the acid class ; of low specific gravity and no 

 great fluidity, and would consequently not spread far from the 



