262 
THE DEVONIAN VOLCANOES 
KOOK V 
<iiid composition of gabbros, and show an entire absence of tlie vesicular 
structure. But no one has yet attempted to separate the two types from 
each other. 
With these rocks are associated abundant diabase-tuffs (schalstein), 
frequently mingled with ordinary non-volcanic detrital matter, and shading 
off into the surrounding grits and slates. There is thus clear evidence of 
the outpouring of basic lavas and showers of ashes during the Devonian 
period in the south-west of England, under conditions analogous to those 
which charactei'ized the deposition of the Devonian system in Nassau and 
the Harz. 
Tlie exact range of these eruptions in geological time has still to be 
ascertained. So far as at present determined, volcanic activity was not 
awakened during the accumulation of the Lower Devonian formations. It 
was not until the sporadic coral-reefs and shell-banks liad grown up, which 
torm the basement limestones of the Middle Devonian gi’oup, that the first 
eruptions took place. As Godwin Austen, Champernowne and Mr. TJssher 
have shown, some of these reefs were overwhelmed with streams of lava or 
buried under showers of ashes. The volcanic discliarges, however, were 
peculiarly local, prolxibly from many .scattered vents, and never on any 
great scale. Some districts remained little or not at all affected by them, 
so tliat the growth of limestone went on without interruption, or at 
least with no serious break. In other areas again the place of the lime- 
stone is taken by volcanic materials. 
The chief epoch of this volcanic action, marked by the “ Ashprington 
Volcanic Series,” appears to have occurred about midway in the Middle 
Devonian period. But in certain districts it extended into Upper 
Devonian time. Intrusive sills of diabase may mark the later phases of the 
volcanic history. But the occurrence of such sills even in the Upper 
Devonian rocks, and the alteration of the strata in contact with them 
(spilosite), point to the continuance or renewal of subterranean disturbance 
even in the later Devonian ages, if not in subsequent geological time. 
That volcanic activity accompanied the deposition of the Carboniferous rocks 
of Devonshire has long been well known (see Chapter xxix.). 
