178 
THE TERTIARY VOLCANOES 
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matter, and now appear as dykes. “ The results above obtained,” he says, 
“ will manifestly hold equally, whether we suppose the uplifted mass acted 
upon immediately through the medium of an elastic vapour or by matter in 
a state of fusion in immediate contact with its lower surface. In the latter 
case, however, this fused matter will necessarily ascend into the fissures, and 
if maintained there till it cools and solidifies, will present such phenomena 
as we now recognize in dykes and veins of trap.” 
The existence of a vast lake or reservoir of molten rock under the 
fissure-region of Britain is demonstrated by the dykes. But, if we inquire 
further what terrestrial operation led to tire uprise of so vast a body of lava 
towards the surface in older Tertiary time, we find that as yet no satisfactory 
answer can be given. 
2. In some districts the dykes can be connected with the gabbros which 
occur as intrusive sills and irregular bosses in the basalt-plateaux and 
among older rocks. The gabbros, however, are traversed by still later dykes, 
which must then be independent of any visible mass of these rocks. The con- 
nection of dykes with the gabbros is what we might naturally expect to find, 
if the more coarsely crystalline rock represents portions of the basic magma 
which consolidated at some depth below the surface. If we could penetrate 
deep enough, it is not improbable that the dykes might be found in large 
measure to shade downward into vast bodies of gabbro. Such a relation 
has been observed in the Yellowstone district, where Mr. Iddings has 
noticed that the centre toward which the dykes of the Old Crandale volcano 
converge is a large mass of granular gabbro, passing into diorite, the dykes 
becoming rapidly coarser in grain as they approach the gabbro-eore. 1 
3. The rise of molten rock in thousands of fissures over so wide a region 
is to my mind by far the most wonderful feature in the history of volcanic 
action in Britain. The great plateaux of basalt, and the mountainous bosses 
of rock by which they have been disrupted, are undoubtedly the most 
obvious memorials of Tertiary volcanism. But, after all, they are merely 
fragments restricted to limited districts. The dykes, however, reveal to us 
the extraordinary fact that, at a period so recent as older Tertiary time, 
there lay underneath the area of Britain a reservoir or series of reservoirs of 
lava, the united extent of which must have exceeded 40,000 square miles. 
That the material of the dykes rose in general directly from below, and 
was not, except locally, injected laterally along the open fissures, may be in- 
ferred, although proof of such lateral injection on a small scale may here and 
there be detected. The narrowness of the rents, and their enormous relative 
length, make it physically impossible that molten rock could have moved 
along them for more than short distances. The usual homogeneous char- 
acter of the dyke-rocks, the remarkable scarcity of any broken-up consoli- 
dated fragments of them immersed in a matrix of different grain, the general 
uniformity of composition and structure from one end of a long dyke to 
another, the spherical form of the amygdales, the usual paucity of fragments 
from the fissure walls — all point to a quiet welling of the lava upward. 
1 Journ. Geol. i. (1893), p. 608. 
