Vol. 52.] BASALT-PLATEAUX OF NORTH-WESTEKN EUROPE. 401 



quently such dislocations give rise to clefts in the cliffs. Some 

 good illustrations of this feature may be noticed on the northern side 

 of the island of Canna, where the highest part of the precipice has 

 been fissured by a series of dislocations, having a hade towards the 

 west and a throw which may in some cases amount to about 20 or 

 25 feet. The cumulative effect of this system of faulting, combined 

 with a gentle westerly dip, is to bring down to the sea-level 

 the upper band of conglomerate which farther east lies at the 

 top of the cliff. Again, the basalt-escarpment on the western side 

 of Skye, from Dunvegan Head to Loch Eynort, is traversed by a 

 number of small faults. On the eastern side of Skye and in Kaasay 

 a series of faults, some of them having perhaps a throw of several 

 hundred feet, has been mapped by Mr. H. B. Woodward. 



The largest dislocation observed by me among the fragments of 

 the basalt-plateaux is that which runs at the back of the Morven 

 outlier, in the west of Argyleshire. 1 It runs from the head of Loch 

 Aline to the mouth of Loch Sunart along the line of valley that 

 contains the salt-water fjord Loch Teacus and the freshwater lakes 

 Loch Durinemast and Loch Arienas. While the Cretaceous deposits 

 and the bottom of their overlying basalts rise but little above the 

 sea-level on the south-western side of this line, they are perched as 

 outliers on hilltops on the north-eastern side, where they rise to 1300 

 feet above the sea. The amount of vertical displacement here 

 probably exceeds 1000 feet. The fault runs in a north-westerly 

 direction, and has obviously been the guiding influence in the erosion 

 of the broad and deep valley which marks its course at the surface. 



To what extent the dislocations that traverse the Tertiary basalts 

 of the Inner Hebrides are to be regarded as comparable to those 

 which in Iceland have been referred to subsidence caused by the 

 tapping and outflow of the lower still liquid parts of lava-sheets 

 must be matter for further enquiry. So far as my own observations 

 have yet gone, the faults do not seem explicable by any mere super- 

 ficial action of the kind supposed. Where they descend through 

 many hundreds of feet of successive sheets of basalt and dislocate 

 the Secondary rocks underneath, they must obviously have been 

 produced by much more general and deep-seated causes. 



It is conceivable that, if these dislocations took place during the 

 volcanic period, they broke up the lava-plains into sections, some 

 of which sank down so as to leave a vertical wall at the surface 

 on one side of the rent, or even to form open ' gjas,' like those of 

 Iceland. But it is noteworthy that the fissures which have been 

 filled with basalt and now appear as dykes, comparatively seldom 

 show any displacement in the relative levels of their two sides. In 

 Iceland, also, the great lava-emitting fissures seem to be in general 

 free from marked displacement of that kind. 



The faults in the Inner Hebrides, so far as I have observed, are 

 all normal, and indicate nothing more than gentle subsidence. But 

 among the Faroe Islands I have come upon several instances of 

 reversed faults, which, in spite of the gentle inclinations of the 



1 This fault was noticed by Prof. Judd traversing the cliffs of the Sound of 

 Mull, and is referred to in iny memoir already cited. 



