8o 
SUBTERRANEAN VOLCANIC ACTION 
BOOK I 
ting the crests of the moixntains and extending as reefs below the level of 
the sea. They are thus exposed in every conceivable divergence of position 
and in endless varieties of enclosing rock. Moreover, they can be shown 
to represent a vast range of geological time. One system of them belongs 
to some remote part of the Archtean periods, another is as young as the 
older Tertiary ages. 
Full details regarding tJiese interesting relics of volcanic activity will 
be given in later chapters, especially in Chapters xxxiv. and xxxv. It 
may suffice here to note that each of the three types of old volcanoes above 
described has, in Eritain, its accompaniment of dykes and veins. The plateaux, 
however, present by far the most abundant and varied development of them. 
The dykes of this series are characterized not only by their prodigious 
numbers in and around some of the plateaux, but by the long distances to 
which they may be traced beyond these limits. They are chiefly found in 
Fig. 32. — Dyke, Vein and Sill. 
The dyke (d) rises along a small fault among sandstones, shales, and ironstones (sh), and gives off a vein (?’) and an 
intrusive sheet or sill (6). 
connection with the Tertiary basalt -plateaux, though the Carboniferous 
andesite -plateaux present a feebler display of them. The Tertiary dykes 
are pre-eminently distinguished by their persistent rectilinear lines, some- 
times for distances of many miles, and their general north-westerly direc- 
tion. They form a vast system extending over an area of some 40,000 
square miles. Throughout that vyide region their persistence of direction 
and of petrographical characters point to the former existence of one or 
more reservoirs of an andesitic and basaltic magma underneath the northern 
half of Britain, and to the ruptui’e of the crust overlying this subterranean 
reservoir by thousands of parallel fissures. They thus constitute perhaps' 
the most astonishing feature in the volcanic history of Tertiary time. 
The dykes and veins connected with the puys are mainly to be found at 
or close to the vents. Not infrequently they traverse the agglomerates of 
the necks, and are sometimes to be traced to a central pipe or core of 
basalt. 
The larger cones are likewise intersected with similar vertical, inclined 
