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.] _Suggested Explanation of the 'Oases.'_ 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.