nwards. These stresses rapidly replace one
another as the satellite travels along. They are resisted by the
inertia of the crust, and are taken up by its elasticity. The
nature of this succession of alternate compressions and
rarefactions in the crust possess some resemblance to those
arising in an earthquake shock.

If we consider the effects taking place laterally to the line of
motion we see that there are no such changes in the nature of the
forces in the crust. At each passage of the satellite the
horizontal tearing stress increases to a maximum, when it is
exerted laterally, along the line passing through the horizontal
projection of the satellite and at right angles to the line of
motion, and again dies away. It is always a tearing stress,
renewed again and again.

This effect is at its maximum along two particular parallel lines
which are tangents to the circle of maximum horizontal stress and
which run parallel with the path of the satellite. The distance
separating these lines depend upon the elevation of the satellite
above the planet's surface. Such lines mark out the theoretical
axes of the "double canals" which future crustal movements will
more fully develop.

It is interesting to consider what the effect of such

185

conditions would be if they arose at the surface of our own
planet. We assume a horizontal force in the crust adequate to set
up tensile stresses of the order, say, of fifteen tons to the
square foot and these stresses to be repeated every few hours;
our world being also subject to the dynamic effects we recognise
in and beneath its crust.

It is easy to see that the areas over which the satellite exerted
its gravitational stresses must become the foci --foci of linear
form--of tectonic developments or crust movements. The relief of
stresses, from whatever cause arising, in and beneath the crust
must surely take place in these regions of disturbance and along
these linear areas. Here must become concentrated the folding
movements, which are under existing conditions brought into the
geosynclines, along with their attendant volcanic phenomena. In
the case of Mars such a concentration of tectonic events would
not, owing to the absence of extensive subaerial denudation and
great oceans, be complicated by the existence of such synclinal
accumulations as have controlled terrestrial surface development.
With the passage of time the linear features would probably
develop; the energetic