CHAPTER XXXYI 
THE PLATEAUX 
Nature and Arrangement of the Rooks: 1 . Lavas. — Basalts, Dolerites, Andesites — 
Structure of the Lavas in the Field — 2 . Fragmental Rocks. — Agglomerates, 
Conglomerates, and Breccias — Tuffs and their accompaniments. 
We liave now to consider the structure and history of those volcanic masses 
which, during Tertiary time, were ejected to the surface within the area of 
the British Islands, and now remain as extensive plateaux. Short though 
the interval has been in a geological sense since these rocks were erupted, it 
has been long enough to allow of very considerable movements of the ground 
and of enormous denudation, as will be more fully discussed in Chapters 
xlviii. and xlix. Hence the superficial records of Tertiary volcanic 
action have been reduced to a series of broken and isolated fragments. 
I have already stated that no evidence now remains to show to what 
extent there were actual superficial outbursts of volcanic material over 
amch of the dyke-region of Britain. The subsequent waste of the surface 
has been so enormous that various lava-fields may quite possibly have 
stretched across parts of England and Scotland, whence they have since 
>een wholly stripped off, leaving behind them only that wonderful system 
°t dykes from which their molten materials were supplied. 
There can be little doubt, however, that whether or not other Phlegrean 
kelds extended over portions of the country whence they have since been 
voin away, the chief volcanic tract lay in a broad and long hollow that 
stretched from the south of Antrim to the Minch. From the southern to 
. le northern limit of the fragmentary lava-fields that remain in this 
^epression is a distance of some 250 miles, and the average breadth of 
found within which these lava-fields are preserved may be taken to range 
lorn 20 to 50 miles. If, therefore, the sheets of basalt and layers of tuff 
extended over the whole of this strip of country, they covered a space of 
some 7000 or 8000 square miles. But they were not confined to the area 
p ie Hritish Islands. Similar rocks rise into an extensive plateau in the 
-roe Islands, and it may reasonably be conjectured that the remarkable 
submarine ridge which extends thence to the North-west of Scotland, and 
parates the basin of the Atlantic from that of the Arctic Ocean, is partly 
