[Footnote 15: The coefficient of contraction of basalt is 0.000006 for 1° F., which would lead to the results given here.]
[Footnote 16: Mr. W.H. Pickering observed clouds on Mars 15 miles high; these are the 'projections' seen on the terminator when the planet is partially illuminated. They were at first thought to be mountains; but during the opposition of 1894, more than 400 of them were seen at Flagstaff during nine months' observation. Usually they are of rare occurrence. They are seen to change in form and position from day to day, and Mr. Lowell is strongly of opinion that they are dust-storms, not what we term clouds. They were mostly about 13 miles high, indicating considerable aerial disturbance on the planet, and therefore capable of producing proportional surface denudation.]
The numerous round dots seen upon the 'canals,' and especially at points from which several canals radiate and where they intersect--termed 'oases' by Mr. Lowell and 'craterlets' by Mr. Pickering may be explained in two ways. Those from which several canals radiate may be true craters from which the gases imprisoned in the heated surface layers have gradually escaped. They would be situated at points of weakness in the crust, and become centres from which cracks would start during contraction. Those dots which occur at the crossing of two straight canals or cracks may have originated from the fact that at such intersections there would be four sharply-projecting angles, which, being exposed to the influence of alternate heat and cold (during day and night) on the two opposite surfaces, would inevitably in time become fractured and crumbled away, resulting in the formation of a roughly circular chasm which would become partly filled up by the debris. Those formed by cracks radiating from craterlets would also be subject to the same process of rounding off to an even greater extent; and thus would be produced the 'oases' of various sizes up to 50 miles or more in diameter recorded by Mr. Lowell and other observers.
_Probable Function of the Great Fissures._
Mr. Pickering, as we have seen, supposes that these fissures give out the gases which, overflowing on each side, favour the growth of the supposed vegetation which renders the course of the canals visible, and this no doubt may have been the case during the remote periods when these cracks gave access to the heated portions of the surface layer. But it seems more probable that Mars has now cooled down to the almost uniform mean temperature it derives from solar heat, and that the fissures--now for the most part broad shallow valleys--serve merely as channels along which the liquids and heavy gases derived from the melting of the polar snows naturally flow, and, owing to their nearly level surfaces, overflow to a certain distance on each side of them.