ickness as compared with
the central mass, which outer layer would have cooled from a highly
heated state to a temperature considerably below the freezing-point, and
this would have been all the time _contracting upon a previously cold,
and therefore non-contracting nucleus._ The result would be that very
early in the process great superficial tensions would be produced, which
could only be relieved by cracks or fissures, which would initiate at
points of weakness--probably at the craterlets already referred to--from
which they would radiate in several directions. Each crack thus formed
near the surface would, as cooling progressed, develop in length and
depth; and owing to the general uniformity of the material, and possibly
some amount of crystalline structure due to slow and continuous cooling
down to a very low temperature, the cracks would tend to run on in
straight lines and to extend vertically downwards, which two
circumstances would necessarily result in their forming portions of
'great circles' on the planet's surface--the two great facts which Mr.
Lowell appeals to as being especially 'non-natural.'

_Symmetry of Basaltic Columns._

We have however one quite natural fact on our earth which serves to
illustrate one of these two features, the direction of the downward
fissure. This is, the comparatively common phenomenon of basaltic
columns and 'Giant's Causeways.' The wonderful regularity of these, and
especially the not unfrequent upright pillars in serried ranks, as in
the palisades of the Hudson river, must have always impressed observers
with their appearance of artificiality. Yet they are undoubtedly the
result of the very slow cooling and contraction of melted rocks under
compression by strata _below and above them_, so that, when once
solidified, the mass was held in position and the tension produced by
contraction could only be relieved by numerous very small cracks at
short distances from each other in every direction, resulting in five,
six, or seven-sided polygons, with sides only a few inches long. This
contraction began of course at the coolest surface, generally the upper
one; and observation of these columns in various positions has
established the rule that their direction lengthways _is always at right
angles to the cooling surface_, and thus, whenever this surface was
horizontal, the columns became almost exactly vertical.

_How this applies to Mars._

One of the features of the surface o