GEOLOGY 
It may be well to state here that the material shot out from a vol- 
cano during an explosive eruption is called tuff (or ‘ash’) in the cases 
where the material falls outside of the crater, quite without regard to 
whether that material is coarse or fine. The material which fills up an 
old vent or ‘neck’ is called agglomerate, whether it is coarse or fine. The 
term /ava is restricted to the floods of rock which have poured out of 
the vents over the surface of the cone; while the same kind of material 
injected below the surface gives rise to a si// if it consolidates in the 
form of a more or less horizontal sheet, and to a dyke if it consolidates 
in a wall-like mass. 
The Cumberland volcanic series consists of rocks of Upper Are- 
nig and Llandeilo age. Lithologically, the greater part of the rocks 
consist of andesites and andesite tuffs, which approach basalts in the 
earlier part. Furthermore, there is a newer and higher volcanic 
group of different lithological character associated with the rocks just 
noticed, and to which fuller reference will need to be made further 
on. 
(4) In the meantime we have to notice a group of rocks which are 
contemporaneous with those of volcanic origin, and which consist mainly 
of sediments. To enable the reader to understand their relation to the 
rocks just noticed, he is asked to bear in mind that a volcano is neces- 
sarily limited in horizontal extent, and that the lava streams which reach 
its flanks, as well of course as the tuffs beyond, will tend to be laid 
down alternately with marine sediments if the volcano is anywhere near 
the sea, and that the relative proportion of volcanic to sedimentary 
matter diminishes as we advance outward from the central area until it 
finally comes to nothing. It must be obvious on reflection that this 
must have been the case as much in connection with the volcanoes of 
the past as it is with those of the present. 
We are therefore quite prepared to find that while the old Cum- 
berland volcano was gradually rearing its cone above the level of the sea 
the deposition of sediment went on contemporaneously on the sea bottom 
outside its flanks. Nearer to the sphere of volcanic action the old sedi- 
ments occasionally received showers of fine tuff which had been wafted 
far out to sea during explosions of a more violent character than usual. 
Such volcanic material thus became mixed in every proportion with 
the sedimentary matter—the proportion of the former to the latter in- 
creasing relatively to the nearness of the cone. To put these statements 
into a terser form, we may say that on the flanks of the volcano there 
was a passage from purely volcanic to purely sedimentary material, 
chiefly through interstratification. 
Hence we may safely conclude that wherever such passage beds 
occur, they mark the seaward flank of the volcano of that particular 
period. 
In north-eastern Cumberland the map shows that there is a narrow 
strip along the foot of the Cross Fell range along which some of the 
oldest rocks of the district have been abruptly elevated to the surface. 
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