geological period, back at least as far as the Lower Silurian, has 
had its voleanoes.! In Britain, for instance, there were active voleanic 
vents in the Lower Silurian period, whence the lavas and tuffs of | 
Snowdon, Aran Mowddwy, and Cader Idris were ejected. The Lower ™ 
Old Red Sandstone epoch was one of prolonged activity in central 
Scotland. The earlier half of the Carboniferous period likewise 
witnessed the outburst of immumerable small volcanoes over the 
same region. During Permian time a few scattered vents existed in 
the south-west of Scotland, and in the epoch of the New Red Sand- 
stone some similar points of eruption appeared in the south of 
England. ‘The older Tertiary ages were distinguished by the 
outpouring of the enormous basaltic plateaux of Antrim and the 
Inner Hebrides. 
In France and Germany likewise paleozoic time was marked by 
the eruption of many diabase and porphyrite lavas, followed in the 
Permian epoch by a great outburst of porphyries, while on the 
other hand the late Tertiary volcanoes of Auvergne, the Eifel, 
Bohemia and Hungary belong almost to the existing period. 
Recent research has brought to light evidence of a long succes- 
sion of Tertiary and post-tertiary volcanic outbursts in Western 
America (Nevada, Oregon, Idaho, Utah, &e.), Contemporaneous 
voleanic rocks are associated with Paleozoic, Secondary and 
Tertiary formations in New Zealand, and volcanic action there is 
not yet extinct. | 
Thus it can be shown that, within the same comparatively 
limited geographical space, volcanic action has been rife at intervals 
during a long succession of geological ages. Even round the sites of 
still active vents traces of far older eruptions may be detected, as in 
the case of the existing active volcanoes of Iceland which rise from 
amid Tertiary lavas and tuffs. Volcanic action, which now mani- 
fests itself so conspicuously along certain lines, seems to have con- 
tinued in that linear development for protracted periods of time. 
The actual vents have changed, dying in one place and breaking 
out in another, yet keeping on the whole along the same tracts. 
§ 5. Causes of Volcanic Action. 
The modus operandi whereby the internal heat of the globe 
manifests itself in voleanic action is a problem to which as yet no 
satisfactory solution has been found. Were this action merely an 
expression of the intensity of the heat, we might expect it to have 
manifested itself in a far more powerful manner in former periods, 
and to exhibit a regularity and continuity commensurate with the 
exceedingly slow diminution of the earth’s temperature. But there 
is no geological evidence in favour of greater voleanic intensity in 
ancient than in more recent periods; on the contrary, it may be 
* The existence of pre-Cambrian Javas has been cited from geveral parts of En: land 
and Wales (see the scction on Archean Rocks in Book Vi.) e ; 

7g 
260 DYNAMICAL GEOLOGY. — [Boox TIL. — 
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