CHAPTER XLIX 
THE SUBSIDENCES ANI) DISLOCATIONS OF THE PLATEAUX 
There can be no doubt that considerable alterations of level have taken 
place over the volcanic areas of North-Western Europe since the eruptions 
that produced the basalt-plateaux. These alterations embrace general and 
local subsidences, and also dislocations by which considerable displacements 
of the crust either in a downward or upward direction have been effected. 
1 SUBSIDENCES 
The mere fact that in many places the lower members of the series of 
terrestrial lavas have been submerged under the sea may be taken to prove 
a subsidence since older Tertiary time. Along the west coast of Skye 
this depression is well shown by the almost entire concealment of the bottom 
of the plateau under the Atlantic. In the Faroe Isles the subsidence has 
advanced still further, for not a trace of the underlying platform on which 
the basalts rest remains above water. In Iceland, too, the complete sub- 
mergence of the base of the Tertiary volcanic sheets points to a widespread 
subsidence of that region. 
Another strong argument in favour of considerable depression 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 which 
are conspicuous on the land are prolonged under the Atlantic. If we are 
correct in regarding the valleys as great lines of subairial erosion, their 
prolongations as fjords and submarine troughs must be considered 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 west end of the Scuir of Eigg. The old 
river-bed and its pitchstone terminate abruptly at the top of a great pre- 
cipice. Assuredly they must once have continued much further 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 
