CHAPTER XLIX
THE SUBSIDENCES AND 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.
i. 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 submergence 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 subærial 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 precipice. 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 would have been filled up by the long-continued outflowings of basalt. Making every allowance for concealed faults and local subsidences, 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 inequalities 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 several districts of the same island, as to indicate that certain portions of the volcanic plain have sunk 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 among the central and northern islands 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 Sandö and Skuö. The highest angle of inclination which I noticed anywhere was at Myggenaes, where the basalts dip E.S.E. at about 15°.
Among the Western Isles, also, where similar variations in the inclination of the basalt-sheets are observable, it might be possible by careful survey to ascertain the probable position of the areas of maximum depression, and to show to what extent differential movements have affected the originally nearly level volcanic floor. It would doubtless be found that everywhere the dominant movement has been one of subsidence. The vast outpourings of lava would tend to leave the overlying crust unsupported, and to cause it to sink into the cavities thus produced.