File:The Quarterly journal of the Geological Society of London (13365333903).jpg

1849..
LYELL ON THE STRUCTURE OF VOLCANOS.
231
would be thrown as much out of the perpendicular as the beds they
mtersect, when the latter were tilted by subsequent movements. The
dikes therefore which have been feeders ought to slope at angles of
Fig, 10. — Volcanic Dikes.
between 23° and 27° to the horizon, whereas in the drawing which I
made of cliffs in the Val del Bove, I have represented them as nearly
all perpendicular.
Had I seen a dike appearing to blend upwards with a sheet of lava,
I should not have inferred any actual connection, unless I could
have scaled the cliff, which is unfortunately inaccessible, and, hammer
in hand, tested every inch of the junction. But had I thus assured
myself of the fact, I should have first inquired whether the dike may
not have sent off veins or branches which had penetrated between
pre-existing parallel strata. If, however, I abandoned this idea as
improbable because a sudden change of direction at right angles could
scarcely occur or very rarely in such intrusive veins, I should have
speculated on the possibility of such dikes having been filled partly
from below and partly from above. After violent eruptions, the
flanks of Etna have been fissured, and a bright light emitted from the
rents has shown that there was incandescent lava below, although it
has sometimes never reached the surface. It is conceivable, therefore,
that lava-currents, descending from the higher and more central parte
of the cone, might in their way fill up some rents of this kind, the tops
of which are often left gaping after eruptions. Such a conjecture would
at least relieve me from the extreme embarrassment in which I am
placed by M. de Beaumont's hypothesis, for I am not called upon in
that case to regard the dikes as the feeders of a series of uniform and
parallel beds of lava, with their accompanying strata of intervening
lapilli and scorise. The whole might then be imagined to have been
poured out or projected from a permanent and powerful central vent,
the eruptions being on a grand scale, so as to allow of a considerable
degree of uniformity in the spreading of the materials over wide areas,
on the sloping side of a great cone inclined at angles between 4 and
10 degrees. A steeper inclination may have been afterwards acquired