400 SIR A. GEIKIE ON THE TERTIARY [May 1 896, 



underlying platform on which the basalts rest remains above water. 

 In Iceland, too, the complete submergence of the base of the 

 Tertiary volcanic sheets points to the widespread subsidence of that 

 region. 



Another strong argument in favour of considerable subsidence may 

 be derived from a comparison of the submarine topography with that 

 of the tracts above sea-level. It is obvious that the same forms of 

 contour as those which are conspicuous on the land are prolonged 

 under the Atlantic. If we are correct in regarding the valleys as 

 great lines of subaerial erosion, their prolongations as fjords and 

 submarine troughs must be regarded as having had a similar origin. 

 We can thus carry down the surface of erosion several hundred feet 

 lower than the line along which it disappears under the waves. 



I know no locality where this kind of reasoning is so impressively 

 enforced upon the mind as the western end of the Scuir of Eigg. The 

 old river-bed and its pitchstone terminate abruptly at the top of a 

 great precipice. Assuredly they must once have continued much 

 farther westward, as well as the sheets of basalt that form the main 

 part of the cliff. Yet the sea in front of this truncated face of rock 

 rapidly deepens to fully 500 feet in some places. Had any such 

 hollow existed in the volcanic period it would have been filled up 

 by the long-continued outflowings of basalt. We can only account 

 for this submarine topography by regarding it as having been 

 carved out, together with the topography of the land, at a time when 

 the level of the latter was at least 500 feet higher than it is now. 



The subsidence which is thus indicated along the whole of the 

 North-west of Europe probably varied in amount from one region 

 to another. We seem to have traces of such an inequality in the 

 varying inclinations of different segments of the basalt-plateaux. The 

 angles of inclination are almost always gentle, but they differ so much 

 in direction from island to island, and even among the districts of 

 the same island, as to indicate that certain portions of the volcanic 

 plain sank rather more than other portions. 



Thus in the Faroe Islands, where the bare cliffs allow the varying 

 angles of inclination to be easily determined, a general gentle dip of 

 the basalts in a south-easterly direction has been noted by previous 

 observers. This inclination, however, is replaced among the southern 

 islands by an equally gentle dip towards the north-east. The 

 centre of depression would thus seem to lie somewhere about Sando 

 and Skuo. The highest angle of inclination which I noticed any- 

 where was at Myggenaes, where the basalts dip E.S.E. at about 15°. 



Though I have not observed any features among the basalt- 

 plateaux that can be compared to the remarkable rifts and sub- 

 sidences of Iceland, it can be shown that these piles of volcanic 

 material have undoubtedly been fractured, and that portions of them 

 have subsided along these lines of dislocation. Careful examination 

 of the basalt-escarpments of the Inner Hebrides discloses the 

 existence of numerous faults which, though generally of small 

 displacement, nevertheless completely break the continuity of all 

 the rocks in a precipice of 700 or 1000 feet in height. Not infre- 



