130 E. H. L. SCHWARZ 
crater-wall has been almost melted down, but still rises slightly above 
the surface of the inundation; good examples are also seen in the 
Mare Tranquilitatis, Fig. 1. 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 certainly has been determined by the melting 
action of the lava of the base. Further, where there are depressions 
in the area in the borders of the maria, the material of which they are 
composed flows into them as a fluid would have done. 
Fic. 1 Fic. 2 
PHOTOGRAPHS OF THE MOON’S SURFACE, AFTER N. S. SHALER 
Fic. 1.—Mare Tranquilitatis with Mare Serenitatis in the upper portion of the 
plate, showing the margin invaded by lava floods. 
Fic. 2,—Mare Serenitatis, showing on the left the large crater Posidonius and 
above it the crater le Monnier which has part of its wall broken down by the lava 
of the mare. 
The great objection to the hypothesis that the maria were formed 
by molten rock produced by the impact of large bodies falling upon 
the surface of the moon is that similar bodies competent to generate 
a great deal of heat have not fallen upon the earth’s surface in the 
time which has elapsed since the beginning of the geological periods; 
there is so far indeed no recorded geological reason for supposing that 
they have ever fallen upon the planet, but it is just such evidence that I 
wish to submit. It must, however, be remembered that the moon’s 
