ation. Vaster still is the valley of the Jordan through
the Sea of Galilee to the Dead Sea, continued by the Wady Arabah to the
Gulf of Akaba, believed to form one vast geological depression or
fracture extending in a straight line for 400 miles.

Thousands of such faults, dykes, or depressions exist in every part of
the world, all believed to be due to the gradual shrinking of the heated
interior to which the solid crust has to accommodate itself, and they
are especially interesting and instructive for our present purpose as
showing the tendency of such fractures of solid rock-material to extend
to great lengths in straight lines, notwithstanding the extreme
irregularity both in the surface contour as well as in the internal
structures of the varied deposits and formations through which they
pass.

_Probable Origin of the Surface-features of Mars._

Returning now to Mars, let us consider the probable course of events
from the point at which we left it. The heat produced by impact and
condensation would be likely to release gases which had been in
combination with some of the solid matter, or perhaps been itself in a
solid state due to intense cold, and these, escaping outwards to the
surface, would produce on a small scale a certain amount of upheaval and
volcanic disturbance; and as an outer crust rapidly formed, a number of
vents might remain as craters or craterlets in a moderate state of
activity. Owing to the comparatively small force of gravity, the outer
crust would become scoriaceous and more or less permeated by the gases,
which would continue to escape through it, and this would facilitate the
cooling of the whole of the heated outer crust, and allow it to become
rather densely compacted. When the greater portion of the gases had thus
escaped to the outer surface and assisted to form a scanty atmosphere,
such as now exists, there would be no more internal disturbance and the
cooling of the heated outer coating would steadily progress, resulting
at last in a slightly heated, and later in a cold layer of moderate
thickness and great general uniformity. Owing to the absence of rain and
rivers, denudation such as we experience would be unknown, though the
superficial scoriaceous crust might be partially broken up by expansion
and contraction, and suffer a certain amount of atmospheric erosion.

The final result of this mode of aggregation would be, that the planet
would consist of an outer layer of moderate th