A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 17 



that sphere. All the facts indicate that these vast sheets of lava did not come 

 from the interior, and that the interior at the time when they were formed was 

 not in a condition to yield any such masses of liquid rock. We are therefore 

 fairly driven to this working hypothesis. In its favor we may adduce the follow- 

 ing considerations : 



The fall of a considerable body or bodies competent by the conversion of its 

 momentum into heat to produce an extensive melting of the lunar surface, would 

 be likely to develop melted lava under conditions quite different from that which 

 is exuded from volcanoes. Assuming that the bolide came upon the surface at 

 planetary velocity and that it was some miles in diameter, the heat due to the 

 arrest of its movement would, we may fairly suppose, convert the whole of the 

 body into a liquid if not into a gaseous state. A like result would occur in 

 the part of the sphere which received the blow. Moreover, for some distance 

 beyond the seat of impact the shearing strains would probably be sufficient to 

 convert much of the material of the surface into the fluid state, with the result 

 that a mass of lava at very high temperature, equal at least to the bulk of the 

 invading body, and probably several times as great, would be sent at the speed 

 determined by the gravitative value of the sphere radially from the point where 

 the impact took place. It seems also, perhaps, a fair supposition that a great 

 collision of this nature would temporarily form a heated atmosphere enveloping 

 the moon, which would serve to delay the cooling of the molten rock until it had 

 time to find its level. Yet the absence of any deposits of these temporarily volatil- 

 ized materials is indicated by the fact that the light streaks are not obscured! 



In favor of the hypothesis above suggested, it may also be said that the evi- 

 dence of melting effected by the material which forms the plains of the maria is 

 considerable at several points, notably in the case of the vulcanoids on the mar- 

 gins of the seas. It seems quite certain that the walls of these craters next the 

 sea have been in some manner effaced by contact with the material which came 

 against it. Again, as in Flamsteed in the Oceanus Procellarum, the crater wall has 

 been almost melted down, but still rises slightly above the surface of the appar- 

 ent inundation. At many points the material forming the mare comes against 

 extended steep-faced cliffs, which have the same general character as the inner 

 slopes of the great craters, where the form of the declivity pretty certainly has 

 been determined by the melting action of the lava at the base. Furthermore, 

 where there are depressions in the area on the borders of the maria, the material 

 of which they are composed flows into them as a fluid would have done. 



It is also to be noted that at many points where the maria come against 

 gently inclined slopes the material of which they are composed appears to have 

 at first flowed over these low but now unsubmerged areas and then retreated from 

 them, leaving them in a measure smoothed as if by the in-filling of their cavities 

 or perhaps by a partial melting of their projecting features. If such apparent 

 inundation really occurred, it may have been brought about by the frontal wave 

 of the lava which mounted, after the manner of those produced by earthquakes in 

 the sea, for some distance above the permanent level of the inundation. 



