5° 
GEOGRAPHY OF OLD VOLCANOES 
BOOK I 
of these changes are indicated in a series of bedded lavas (1). The lower 
part of the diagram shows the dying out of a bed of river gravel (g) 
against the sloping end of a lava-stream, and the sealing up of this inter- 
calation by a fresh outpouring of lava. Higher up in the diagram a section 
is shown of a gully or ravine which has been cut out of the lavas by a 
stream, and has become choked up with water-worn detritus. Subsequent 
outhows of lava have rolled over this channel and sealed it up. Examples 
of such intercalations of lava with old river deposits, and of the burying 
of water-courses, will be cited in the account of the Tertiary volcanic plateau.x 
of Britain in Chapter xxxviii. 
4. Terreatrml Eruptions . — That volcanoes in former times broke out on 
land as well as in water may readily be expected. But it is obvious that 
the proofs of a terrestrial origin may not be always easy to obtain, for every 
land-surface is exposed to denudation ; and thus the relics of the eruptions 
of one age may be effaced by the winds, rains, frosts and rivers of the next. 
Tn assigning any volcanic group to a terrestrial origin, we may be guided 
partly by negative evidence, such as the absence of all trace of marine 
organisms in any of the sedimentary layers associated with the group. But 
such evidence standing by itself would not be satisfactory or sufficient. If, 
however, between the sheets of lava there occur occasional depressions, 
filled with hardened sediineirt full of land-plants, with possiblj' traces 
of insects and other terrestrial organisms, we nray with some confidence 
infer that these silted-up hollows represeirt pools or lakes that gathered 
on the surface of the lava-sheets, and into which tire vegetation of the 
surrorrirdirrg ground was blowir or washed. Bain fallirrg orr tire rugged 
sirrface of a lava-field worrld naturally gather into pools and lakes, as the 
bottoms of the hollows became “ puddled ” by the gradual decay of the rock 
and the washing of fine silt into the crevices of the lava. 
Again, it nray be expected that prolonged exposure to the air would give 
rise to disiirtegratioir of the lava 
and to the consequeirt fortrra- 
tion of soil. Terrestrial vegeta- 
tion would naturally spriirg up 
on such soil ; trees might take 
root upoir it. Heirce, if airother 
lava-flood deluged the surface, 
the soil and its vegetable mantle 
would be eirtonrbed under the 
molten rock. 
These geological changes are represented diagramirratically in Fig. 21. 
Two hollows among the lavas are there showir to have beerr filled with silt, 
iircludiirg srrccessive layers of vegetation now corrverted irrto coal. Oire of 
the soils (.S-) is marked between the lavas, aird the charred strrrrrp of a tree 
with its roots still in another layer of soil higher up is seen to have been 
engulphed in the overlying sheet of melted rock. 
Admirable illustrations of this succession of events are to be encountered 
Fig. 21. — Duagrain illustrating volcanic eruptions on a 
land-surface. 
