CHAPTER XLI 
THE ERUPTIVE VENTS OF THE BASALT-PLATEAUX 
Vents filled with Basalt or other Lava-form Rock — Vents filled with Agglomerate 
It is one of the most interesting points in the Tertiary volcanic history 
that, in spite of the enormous geological revolutions that have passed since 
they became extinct, the sites of many scattered vents can still be recog- 
nized. A far greater number must lie buried under the basalts, and of 
others the positions are concealed by the sea, which now covers so large 
an area of the old lava-fields. Nevertheless, partly within the area of the 
plateaux, but still more on the surrounding tracts from which the basalts 
have been removed by denudation, the traces of unmistakable vents of dis- 
charge may be recognized amid the general wreck. 
In Britain and the Faroe Isles, it is chiefly along the coast-line that the 
process of denudation has revealed the volcanic vents of Tertiary time. The 
interior of the country is often loaded with peat, covered with herbage, or 
strewn with glacial detritus ; and even where indications of the vents are 
to be detected, it is not always possible to ascertain their true limits 
and connections. But where the structure of the plateaux has been laid 
bare along ranges of rocky precipice, the vents have sometimes been so 
admirably dissected by the sea that every feature of their arrangements can 
be satisfactorily determined. 
As the actual physical connexion of these volcanic orifices with the 
plateaux has been in most cases removed by denudation, we can usually only 
by inference place them in what was probably their true relation to the 
plateau-eruptions. Those which project from the surface of the plateaux 
must, of course, be younger than the basalts through which they rise ; how 
much younger we cannot tell. They may possibly be later than any of the 
plateau-sheets ; they may even belong to a subsequent and waning condition 
of volcanic action. On the other hand, the vents which can now be traced 
outside of the present limits of the edges of the plateaux may, like those 
just mentioned, be younger than the basalt-sheets, or, on the contrary, they 
may he records of a period of eruptivity anterior to the emission of any of 
the rocks of the plateaux, and may have been deeply buried under a mass 
of basalt-beds subsequently removed. Positive demonstration is, from the 
