ut the interior
portion still continuing to contract, owing to the conduction of the
heat towards the exterior, would tend to enter upon a condition of
tension, as becoming too small for the interior molten magma; and such a
state of tension would tend to produce rupture of the interior part. In
this manner fissures would be formed into which the molten matter would
enter; and if the fissures happened to extend to the surface, owing to
weakness of the crust or flexuring of the strata, or other cause, the
molten matter would be extruded either in the form of dykes or volcanic
vents. In this way we may account for the numerous dykes of trap by
which some volcanic districts are intersected, such as those of the
north of Ireland and centre of Scotland.

From the above considerations, it follows that the earth's crust must be
in a condition both of pressure (or lateral thrust) towards the exterior
portion, and of tension towards the interior, the former condition
resulting in faulting and flexuring of the rocks, the latter in the
formation of open fissures, through which lava can ascend under high
pressure. These operations are of course the attempt of the natural
forces to arrive at a condition of equilibrium, which is never attained
because the processes are never completed; in other words, radiation and
convection of heat are constantly proceeding, giving rise to new forces
of thrust and tension.

It now remains for us to consider what may be the condition of the
interior molten magma; and in doing so we must be guided to a large
extent by considerations regarding the nature of the extruded matter at
the surface.

(_i._) _Relative Densities of Lavas._--Now, observation shows that, as
bearing on the subject under consideration, lavas occur mainly under two
classes as regards their density. The most dense (or basic) are those in
which silica is deficient, but iron is abundant; the least dense (or
acid) are those which are rich in silica, but in which iron occurs in
small quantity. This division corresponds with that proposed by Bunsen
and Durocher[6] for volcanic rocks, upon the results of analyses of a
large number of specimens from various districts. Rocks may be thus
arranged in groups:

(1) _The Basic_ (Heavier)--poor in silica, rich in iron; containing
silica 45-58 per cent. Examples: Basalt, Dolerite,
Hornblende rock, Diorite, Diabase, Gabbro,
Melap