128 EA .OL SCHWARZ 
scale of dimensions corresponding with antiquity in the craters, the 
larger being the older and the smaller ones successively younger; 
this we can establish both from the fact that the smaller ones often 
breach the larger ones, as well as from the freshness of the rocks 
about the smaller ones as compared with those surrounding the larger 
ones; for although there is no atmospheric weathering in the moon, 
the alternate heating and cooling of the surface brings about a certain 
amount of alteration which in time produces sufficient effect to be 
clearly noticed through the great telescopes. 
These craters do not belong to volcanoes such as exist on the 
earth but resemble rather the outbursts ofentangled molten matter 
during the final consolidation of the moon, according to Professor T. 
C. Chamberlin’s view,’ and on such a theory the graduation in size 
is well accounted for. But after this stage was over, when the forces 
which brought the molten material from the interior to the surface 
had become spent, no matter what their actual nature was, then the 
surface of the moon was deluged with floods of lava, which, over tracts 
many hundreds and thousands of square miles in area, obliterated 
all pre-existing features and in their margins invaded and ruined 
the craters which stood in the path of the molten liquid. ‘These 
Maria or dark patches of the moon occupy roughly one-third of the 
visible portion, and as seen through the great telescopes which bring 
the moon’s surface to within 4o miles of the observer, their margins 
show that the material of which they are composed flowed in upon the 
rough ground as very liquid lava would do. It fills in the lower 
ground forming numerous bays, and in many instances, as is the case 
of the crater Doppelmeyer, it distinctly appears to have melted down 
the side of the crater-wall next to it and to have filled in the cavity to 
its own level. This feature is not confined to any one spot of any 
one mare, but is to be noticed throughout the several thousand miles 
of the extent of the margins, and leads one to the conclusion that the 
maria were formed by a once fluid matter of the sea inundating firm 
land. ‘The quantity of igneous matter was very great, and in each 
mare or sea it seems to have appeared all at once, there being no 
mark of successive flows such as compose the extensive lava fields of 
the earth. The lava of the several maria never overlap, although the 
2’T. C. Chamberlin and R. D. Salisbury, Geology, II, p. 105. 
