c, fluctuating in
intensity, with longer or shorter periods of repose, shifting in
position, becoming extinct here, appearing in new activity or for the
first time there. We have an adequate solution of the before
inexplicable fact of their propinquity, and yet want of connection. We
have an adequate cause for the fusion of rock at local points without
resorting to the baseless hypothesis of perennial lakes of lava, etc.

For the first time, too, we discern a true physical cause for earthquake
movement, where volcanic energy does not show itself. The crushing of
the world's solid shell, whether thick or thin, goes on _per saltum_ and
at ever-shifting places, however steadily the tangential pressures
producing it may act. Hence crushing _alone_ may be shown to develope
amply sufficient impulse to produce the most violent Earthquakes,
whether they be or be not at a given place or time connected with
volcanic outburst or possible injection, or with tangential pressures,
enough still, in some cases, to produce partial permanent elevation.

When subterraneous crushing takes place, and the circumstances of the
site do not permit the access of water, there may be Earthquake, but can
be no Volcano; where water is admitted, there may be both.

And thus we discern why there are comparatively few submarine
Volcanoes, the floor of the ocean being, on the whole,
water-tight--"puddled," as an engineer would say, by the huge deposit of
incoherent mud, etc., that covers most of it, and probably having a
thicker crust beneath it than beneath the land.

We see, moreover, that the geological doctrine of absolute uniformity
cannot be true as to Vulcanicity, any more than it can for any other
energy in play in our world. Its development was greatest at its
earliest stages, when the great masses of the mountain chains were
elevated. It is even now--though as compared to men's experience, and
even to all historic time, apparently uniform and always the same--a
decaying energy.

The regimen of our planet as part of the Cosmos, which seems to some
absolute (and presented to Playfair no trace of a beginning nor
indication of an end), is not absolute, and only seems to us to be so
because we see so little of it, and of its long perspective in time.
This the now established doctrine of the conservation of energy renders
certain.

With this source for volcanic heat, too, in our possession, we can look
from our own world to others, and predic