258 
THE TERTIARY VOLCANOES 
BOOK VIII 
same vertical wall of rock some of the basalts die out to the south, others 
to the north, while occasionally a shorter sheet may be seen to disappear in 
both directions as if it were the end of a stream that flowed at right angles 
to the others (Fig. 290). 
The more the basalt-plateaux of Britain and the Faroe Islands are 
studied, the more certain 
does the conclusion be- 
come that these wide- 
spread sheets of lava 
never flowed from a few 
large central volcanoes 
of the type of Etna 
Fig. 290. — Lenticular lavas east side of Svino, Faroe Isles. or Vesuvius, but were 
emitted from innumer- 
able minor vents or from open fissures. In a later chapter an account 
will be given of the vents, which may still be seen under the overlying 
sheets of basalt, and, in particular, a remarkable group in the Faroe Islands 
will be described. 
The occurrence of tuffs, leaf-beds and thin coals between the plateau- 
basalts of the Faroe Islands has long been known. These stratified deposits 
are well seen in the island of Sudero, where they serve to divide two distinct 
series of basalts, like the iron-ore and its accompaniments in Antrim. As 
a characteristic illustration of the same diversity 
of deposits observable between the lava-sheets of 
the basalt -plateaux of the British Isles I give 
here a section exposed on the east side of this 
island — a locality often visited and described in 
connexion with its coal-seams (Fig. 291). At the 
base lies a sheet of basalt («) with an irregularly 
lumpy upper surface. It may be remarked that 
the lower group of basalts is marked by the occur- 
rence of numerous columnar sheets, some of them 
possibly sills, and also more massive, solid, and 
durable basalts than the sheets above. The lowest 
of the intercalated sediments are light-coloured 
clays, passing down into dark nodular mudstone 
and dark shale, the whole having a thickness of at 
least 20 feet (&). These strata are succeeded by (e) 
pale clays with black plant-remains, about three 
feet thick. Immediately above this band comes 
the coal or coaly layer (cl), here about six inches 
thick, which improves in thickness and quality further inland, where it has 
been occasionally worked for economic purposes. A deposit of green and 
lirown volcanic mudstone (e), twelve feet in thickness, overlies the coal and 
passes under a well-bedded granular green tuff and mudstone three feet thick 
(/). The uppermost band is another volcanic mudstone (g) four feet in 
