194 
THE TERTIARY VOLCANOES 
BOOK VIII 
tion not of single beds only, but of a whole series of basalts. Thus, at 
Ballyeastle, the group of lavas known as the Lower Basalts, which underlie 
the well-known horizon of iron-ore, are at least 350 feet thick. But, as we 
trace them westwards, bed after bed thins out until, a little to the west of 
Ballintoy, in a distance of only about 6 miles, the whole depth of the group 
has diminished to somewhere about 40 feet. A decrease of more than 300 
feet in six miles or 50 feet per mile points to considerable inequalities in 
the accumulation of the lavas. If the next series of flows came from 
another vent and accumulated against such a gentle slope, it would be 
marked by a slight unconformability. Structures of this kind are much 
rarer than we should expect them to be, considering the great extent to 
which the plateaux have been dissected and laid open in cliff-sections. 
The basalt-plateau of the Faroe Islands exhibits witli remarkable clear- 
ness the lenticular character of the basalt-sheets, and a number of examples 
will be cited in the description of that region to be given in Chapter xxxix. 
In these northern climes vegetation spreads less widely over rock and slope 
than it does in the milder air of the Inner Hebrides. Hence the escarp- 
ments sweep in precipices of almost hare rock from the level of the sea up 
to the serrated crests of the islands, some 2000 feet in height. Each 
individual bed of basalt can thus be followed continuously along the fjords, 
and its variation or disappearance can be readily observed. Coasting along 
these vast natural sections, we readily perceive that, as among the Western 
Isles, the successive sheets of basalt have proceeded from no one common 
centre of eruption. They die out now towards one quarter, now towards 
another, yet everywhere retain the universal regularity and gentle inclina- 
tions of the whole volcanic series. 
ii. FRAGMENTAL ROCKS 
While the plateaux are built up mainly of successive flows of basaltic 
lavas, they include various intercalations of fragmental materials, which, 
though of trifling thickness, are of great interest and importance in regard 
to the light which they cast on the history of the different regions during 
the volcanic period. I shall enumerate the chief varieties of these rocks 
here, and afterwards give fuller details regarding their stratigraphical 
relations and mode of occurrence in connection with the succession of beds 
in each of the plateaux. 
(a) Volcanic Agglomerates. — In the tumultuous unstratified masses 
of fragmentary materials which fill eruptive vents in and around the 
plateaux, the stones, which vary in size up to blocks several feet in 
diameter, consist for the most part of basalts, often highly slaggy and 
scoriaceous. They include also fragments of different acid eruptive rocks 
(generally felsitic or rhyolitic in texture), with pieces of the non-volcanic rocks 
through which the volcanic pipes have been drilled. The paste is granular, 
dirty-green or brown in colour, and seems generally to consist chiefly of com- 
minuted basalt. As in the Carboniferous and Permian necks, the Tertiary 
