But Johnstone Stoney's theory offers another method by which they could have escaped, through evaporation and the gradual flight of the molecules into open space.

Possibly both methods have been in operation, a portion of the constituents of the former atmosphere and oceans having entered into chemical combinations in the lunar crust, and the remainder having vanished in consequence of the lack of sufficient gravitative force to retain them.
But why, it may be asked, should it be assumed that the moon ever had things which it does not now possess?
Perhaps no entirely satisfactory reply can be made.

Some observers have believed that they detected unmistakable indications of alluvial deposits on lunar plains, and of the existence of beaches on the shores of the "seas." Messrs.

Loewy and Puiseux, of the Paris Observatory, whose photographs of the moon are perhaps the finest yet made, say on this subject: "There exists, from the point of view of relief, a general similarity between the 'seas' of the moon and the plateaux which are covered to-day by terrestrial oceans.

In these convex surfaces are more frequent than concave basins, thrown back usually toward the verge of the depressed space.