ermed
'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.

_Suggested Origin of the Blue Patches._

These heavy gases, mainly perhaps, as has been often suggested,
carbon-dioxide, would, when in large quantity and of considerable depth,
reflect a good deal of light, and, being almost inevitably dust-laden,
might produce that blue tinge adjacent to the melting snow-caps which
Mr. Lowell has erroneously assumed to be itself a proof of the presence
of liquid water. Just as the blue of our sky is undoubtedly due to
reflection from the ultra-minute dust particles in our higher
atmosphere, similar particles bro