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Parr I. Sect. i. § 5.) CAUSES OF VOLCANIC ACTION. 261 
doubted whether any of the Paleozoic volcanoes ‘equalled in 
magnitude those of Tertiary and perhaps even post-tertiary times. On 
the other hand, no feature of volcanic action is more conspicuous than 
its spasmodic fitfulness. 
As physical considerations negative the idea of a comparatively 
thin crust surmounting a molten interior whence volcanic energy 
might be derived (ante, p. 49), geologists have found themselves 
involved in great perplexity to explain volcanic phenomena, for the 
production of which a source of no great depth would seem to be 
necessary. ‘They have supposed the existence of pools or lakes of 
liquid lava lying beneath the crust, and at an inconsiderable depth 
from the surface. Some have appealelto the influence of the con- 
traction of the earth’s mass, erroneously assuming the contraction to 
be now greater in the outer than in the inner portions, and that the 
effect of this external contraction must be to squeeze out some of 
the internal molten matter through weak parts of the erust. Cordier, 
for exumple, calculated that a contraction of only a single millimetre 
(about -4,th of an inch) would suffice to force out to the surface lava 
enough for 500 eruptions, allowing 1 cubic kilometre (about 1300 
million cubic yards) for each eruption. 
That volcanic action is one of the results of terrestrial contraction 
can hardly be doubted, though we are still without satisfactory data 
as to the connection between the cause and the effect. It will be 
_ observed that volcanoes occur chiefly in lines along the crests of 
terrestrial ridges. There is evidently therefore a connection between 
the elevation of these ridges and the extravasation of molten rock at 
the surface. The formation of continents and mountain chains has 
already been referred to as probably consequent on the subsidence 
and readjustment of the cool outer shell of the planet upon the hotter 
and more rapidly contracting nucleus, Every such movement, by 
relieving pressure on regions below the axis of elevation, will tend to 
bring up molten rock nearer the surface, and thus to promote the 
formation and continued activity of volcanoes. 
The fissure eruptions wherein lava has risen through innumerable 
rents in the ground across the whole breadth of a country, and has 
been poured out at the surface over areas of many thousand square 
miles, flooding them sometimes to a depth of several thousand 
feet, undoubtedly prove that molten rock existed at some depth 
over a large extent of territory, and that by some means still 
unknown, it was forced out to the surface (ante p. 255). In inves- 
tigating this subject it would be important to discover whether any 
evidence of great terrestrial crumpling or other movement of the 
crust ean be ascertained to have taken place about the same geo- 
logical period as a stupendous outpouring of lava—whether, for ex- 
ample, the great lava fields of Idaho may have had any connection 
with contemporaneous flexure of the North American mountain 
system, or whether the basalt plateaux of Antrim, Scotland, Faroe 
and Iceland may possibly have been in their origin sympathetic with 
