66 MR. A. HARKER ON THE GEOLOGICAL [Feb. I906, 



than that implied in this laying bare of the deeper rocks. # In the 

 precipitous sea-cliffs of Mull, Eigg, Canna, and the northern and 

 western coasts of Skye, the truncated edges of the basalts and 

 dolerites are exposed in sections 500 to 900 feet high. It is clear 

 that the horizontal extension of the rocks has once been far wider 

 than the present land-surface ; and it is highly probable that the 

 stratiform basic rocks of the several islands, now divided by 

 soundings of 50 to 100 fathoms, are merely relics of one continuous 

 tract. Faulting and differential subsidence have played an 

 important part in producing the actual map of these western isles ; 

 but we know of no such movements later than a somewhat early 

 epoch in the phase of minor intrusions, and a truly surprising work 

 must be credited to the co-operation of subaerial and marine erosion, 

 perhaps with some assistance from ice. 1 The basaltic lavas being 

 of subaerial outpouring, their destruction must have begun as soon 

 as they were formed ; and the upper flows, without the protection 

 of dolerite-sills, might be destroyed with comparative rapidity ; 

 but it is none the less manifest that by far the greater part of the 

 erosion was accomplished subsequently. It was not only later 

 than the sills, but later than all the basic dykes, which occur up to 

 the highest summits : that is, it was subsequent to the cessation 

 of igneous activity. On the other hand, it was practically com- 

 plete, except in the higher mountains and possibly in some of the 

 sounds, before the advent of Glacial conditions. Between these 

 limits we have to find scope for the agents of erosion to accomplish 

 their work ; and, even with all allowance for the land standing 

 then at a higher elevation than now above sea-level, the results are 

 not a little surprising. 



Bearing these considerations in mind, we see that the lava-flow 

 hypothesis of the Sgurr of Eigg confronts us with very great 

 difficulties. It takes this small area quite out of relation with 

 the rest of the region, and demands for it a chronology irreconcilable 

 with that deduced elsewhere. The valley-bottom, now the highest 

 ground on the island, appears thus as a relic of a vanished 

 topography, of which we have no other trace. The carving out of 

 such a valley, down to within 600 feet of the base of the basalts, 

 implies a remarkable amount of erosion, which must be compressed 

 within extremely narrow time-limits. Eor, by the same reasoning 

 as before, it must have been in the main subsequent to the intrusion 

 of the dolerite-sills and dykes, and, ex hypothesis it was completed 

 prior to the pitchstone-epoch. The greater part of the work must 

 in this case have been accomplished in a small fraction of the time 

 covered by the phase of minor intrusions. Nor can we evade the 

 difficulty by supposing the pitchstone of the Sgurr to represent an 

 isolated episode, of much later date than all other igneous rocks in 

 the region; for the evidence of immense erosion subsequent to 



1 The soundings on the Admiralty-Charts indicate well-marked basins in 

 numerous places. Good examples occur in the Sounds of Raasay and 

 Applecross, on either side of Raasay and Rona. 



