Page:Is Mars habitable - Wallace 1907.djvu/114

VII.] 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.

One of the features of the surface of Mars that Mr. Lowell describes with much confidence is, that it is wonderfully uniform and level, which of course it would be if it had once been in a liquid or plastic state, and not much disturbed since by volcanic or other internal movements. The result would be that cracks formed by contraction of the hardened outer crust would be vertical; and, in a generally uniform material at a very uniform temperature, these cracks would continue almost indefinitely in straight lines. The hardened and contracting surface being free to move laterally on account of there being a more heated and plastic layer below it, the cracks once initiated above would continually widen at the surface as they penetrated deeper and deeper