448 
THE TERTIARY VOLCANOES 
BOOK VIII 
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 indi- 
cate 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 Sando and 
Skub. 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 inclina- 
tion of the basalt-sheets are observable, it might be possible by careful 
survey to ascertain the probable position of the areas of maximum depres- 
sion, 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. 
Perhaps the most extensive subsidence of this kind, at least that which 
admits of most satisfactory investigation, because it still remains above 
sea-level, is displayed by the vast hollow in the Antrim plateau, which 
embraces the basin of Lough Neagh and the valley of the Lower Bann. 
This depression measures about 60 miles in length by about 20 in breadth. 
Its axis follows the N.N.W. trend so characteristic of the volcanic features 
of Tertiary time. The depression may be said to involve the entire 
basaltic plateau of Antrim, for with the exception of a few insignificant 
areas along the borders, especially on the east side between Larne and 
Oushendall, the whole region slopes inward from its marginal line of escarp- 
ments, which reach heights of 1800 feet and upwards, towards the great 
hollow in its centre (see Map VII.). 
Lough Neagh, which occupies the deepest part of this hollow, and covers 
about one-eighth of the whole area of subsidence, is the largest sheet of fresh 
water in the British Isles, for it exceeds 150 square miles in extent of 
surface. Yet, for its size, it is one of the shallowest of our lakes, its average 
depth being less than 40 feet. Its shallowness, compared with its wide 
