t towards one point only of the compass, as would
happen if it had descended the sloping sides of a cone. This argument is
ingeniously and plainly put in the following terms:--"Had the melted
matter poured down an inclined plane, after issuing from a rent, the
sheet of lava would, after consolidation, have formed an elbow with the
dike, like the upper bar of the letter F, instead of extending itself on
both sides like that of a T."[578] It is also contended that a series of
sheets of lava, formed on a conical or dome-shaped mountain, would have
been more numerous at points farthest from the central axis, since every
dike which had been the source of a lava-stream, must have poured its
contents downwards, and never upwards.

[Illustration: Fig. 54.

Dikes as they would now appear had they been originally perpendicular.]

In reference to the facts here stated, I may mention that the dikes
which I saw in the Val del Bove were either vertical, or made almost all
of them a near approach to the perpendicular, which could not have been
the case had they been the feeders of horizontal beds of lava, and had
they consequently joined them originally at right angles, for then the
dikes, as at _a_, _b_, _c_, fig. 54, ought subsequently to have acquired
a considerable slope, like the beds which they intersect. I may also
urge another objection to the views above set forth, namely, that had
the dikes been linear vents, or orifices of eruption, we must suppose
the inter-stratified scoriae and lapilli, as well as the lavas, to have
come out of them, and in that case the irregular heaping up of
fragmentary matter around the vents would, as before hinted, have
disturbed that uniform thickness and parallelism of the beds which M. de
Beaumont describes.

If, however, some of the sheets of lava join the dikes in such a manner,
as to imply that they were in a melted state simultaneously with the
contents of the fissures,--a point not easily ascertained, where the
precipices are for the most part inaccessible,--the fact may admit of a
different interpretation from that proposed by the French geologists.
Rents like those before alluded to (p. 399), which opened in the plain
of S. Lio in 1669, filled below with incandescent lava, may have lain in
the way of currents of melted matter descending from higher openings. In
that case, the matter of the current would have flowed into the fissure
and mixed with the lava at its bottom. Numerous open r