CHAP. IV 
LACUSTRINE AND FLUVIATILE ERUPTIONS 
49 
of the limestone, lint that the sea .still abounded in life is shown by the 
numerous organisms imbedded in the second stratified band of tuff. At 
last an access of volcanic vigour gave vent to a streani of slaggy lava, which 
rolled over the sea-bottom and solidified in the thick sheet of amydaloidal 
basalt marked B. This outliow was followed by a further discharge of 
ashes and stones, which, from their absence of stratification, may be supposed 
to have been the result of a single explosion, or at least to have fallen too 
rapidly for the marine currents to rearrange them in layers. When the 
water cleared, the abundant sea-creatures returned, and from their crowded 
remains limestone once more gathered over tlie bottom. Yet the volcanic 
history had not then reached its close, for again there came a discharge of 
ashes, followed by tlie outpouring of a second lava, which consolidated as a 
sheet of columnar basalt (!’/). 
It is not necessary, in order to prove the eruptions to have been sub- 
marine, that organic remains should be found in the tuffs or between them. 
If the volcanic ejections are intercalated among strata which elsewhere can 
be proved to be marine, their discharge must obviously have taken place 
under the sea. The vent that discharged them may have raised its head 
above the sea-level, but its lavas and tuffs were spread out over the adjoin- 
ing sea-fioor. 
2. Lacustrine Eruptions — The same line of evidence furnishes proof 
that some volcanoes arose in inland sheets of water. If their products are 
interstratified among sandstones, gravels and shell-marls, wherein the remains 
of land-plants, insects and lacustrine shells, are preserved, we may be con- 
fident that the eruptions took place in or near to some lake-basin. The 
older lavas and tuffs of Central France supply an instructive example of such 
an association. In Britain, the abundant and extensive outpouring of lavas 
and tuffs during the time of the Lower Old Bed Sandstone probably occurred 
in large lakes. Among the sediments of these bodies of water, interstratified 
between the volcanic sheets, remains of land-plants are abundant, together 
with, here and there, those of myriapods washed down from the woodlands, 
and of many forms of ganoid fishes. 
3. Fluviatik EnqMons . — Volcanoes have sometimes arisen on river- 
plains or on the edges of valleys ( .d F LJjjH 
I ^ 
and 
gorges, and have poured 
out their lavas and discharged 
their ashes over the channels 
or alluvial lands of the streams. 
Volcanic materials, usurping the 
water -channels, bury or are 
interstratified with fiuviatile 
sand or shingle, containing per- 
haps remains of the vegetation 
or animal life of the surrounding land. There may thus be a constant 
shifting of the river-courses, and a consequent deposit of fiuviatile sediment 
at many successive levels among the lavas and tuffs. In Fig. 20 some 
VOL. I E 
Eig. 20. — Diagram illu.stratiiig volcanic eruptions on a 
river-plain. 
