48 
GEOGRAPHY OF OLD VOLCANOES 
BOOK I 
chronology of the volcanic rocks of the British Isles has been worked out, 
and that the following chapters luive been arranged. 
V. HOW THE PHYSICAL GEOGRAPHY ASSOCIATED WITH ANCIENT 
VOLCANOES IS ASCERTAINED 
Wliile the materials erupted from old volcanic vents tell plainly enough 
their subterranean origin, they may leave us quite in the dark as to the 
conditions under which they were thrown out at the surface. Yet a careful 
examination of the strata associated with them may throw much light 
on the circumstances in which the eruptions took place. Many of the 
results of such examination will he given in subsequent chapters. I will 
here submit illustrations of how four different phases of physical geography 
during former volcanic eruptions may be satisfactorily determined. 
1. Siihinarine Eruptions . — As by far the largest accessible part of the 
crust of the earth consists of old marine sediments, it is natural that the 
volcanic records preserved in that crust should 
be mainly those of submarine eruptions. That 
many lavas during the geological past were poured 
out upon the sea-bottom is i)lainly shown by the 
thick beds of marine organisms which they have 
overspread and which lie above them (Fig. 19). 
In Central Scotland, for example, sheets of basalt 
have flowed over a sea-bottom on which thick 
groves of crinoids, bunches of coral and crowds 
of sea -shells were living. Not less striking is 
the evidence supplied by bands of tuff. Around 
Limerick, for instance, the thick Carboniferous 
Limestone is interrupted by many thin layers 
of tuff marking intervals when showers of volcanic 
dust fell over the sea- bottom, killing off the 
organisms that lived there. But the limestone 
that overlies these volcanic intercalations is again 
crowded with fossils, proving that the crinoids, 
corals and shells once more spread over the place 
of ijivas tiiid tuffs with liuic- and houiTshed as abundantly as evei above the 
•stones and shales full of marine 
organisms. ’ . 
The accompanying diagram (lig. 19) illustrates 
these statements. At the bottom a thick mass of limestone (1) full of crinoids, 
corals, brachiopods and other marine organisms bears witness to a long time 
of repose, when the clear sea -water teemed with life. At last a volcanic 
explosion took place, which threw out the first seam of tuff (t). But this 
was only a transient interruption, for the accumulation of calcareous sediment 
was immediately resumed, and the next band of limestone was laid down. 
Thereafter a more prolonged or vigorous eruption ejected a larger mass of 
<lust and stones, which fell over the bottom find prevented the continuation 
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