1874.] The Past History of ouv Moon. 315 
would be full to the brim (or nearly so), at all times, with 
occasional overflows; and as a writer who has recently 
adopted this theory has remarked—‘‘ We should ultimately 
have a large central lake of lava surrounded by a range of 
hills, terraced on the outside,—the lake filling up the space 
taey enclosed.” 
The crust might burst in the manner here considered, at 
several places at the same—or nearly the same—time, the 
range of the radiating fissures depending on the extent of 
the underlying lakes of molten matter thus finding their 
outlet; or there might be a series of outbursts at widely 
separated intervals of time, and at different regions, gradually 
diminishing in extent as the crust gradually thickened and 
the molten matter beneath gradually became reduced in 
relative amount. Probably the latter view should be ac- 
cepted, since if we consider the three systems of radiations 
from Copernicus, Aristarchus, and Kepler, which were 
manifestly not formed contemporaneously, but in the order 
in which their central craters have just been named, we see 
that their dimensions diminished as their date of formation 
was later. According to this view we should regard the 
radiating system from Tycho as the oldest of all these 
formations. 
At this very early stage of the moon’s history, then, 
we regard the moon as a somewhat deformed spheroid, 
the regions whence the radiations extended being the 
highest parts, and the regions farthest removed from the 
ray centres being the lowest.* To these lower regions what- 
ever was liquid on the moon’s surface would find its way. 
The down-flowing lava would not be included in this descrip- 
tion, as being rather viscous than liquid; but if any water 
at least lay melted rock of the same nature, nothing is more natural than that 
the contraction of the crust should cause great overflows of lava, which would 
spread far and wide; the outside portions would cool, but those near the 
centre of disturbance would be kept at. their original temperature, and the 
tendency would be (as is so often noticed in eruptions) to melt the already 
solidified rock with which it was in conta@, and thus the orifice would become 
wider.” 
* Where several ray centres are near together, a region dire@ly between 
two ray centres would be at a level intermediate between that of the ray 
centres and that of a region centrally placed within a triangle or quadrangle of 
tay centres; but the latter region might be at a higher level than another very 
far removed from the part where the ray centres were near together. For in- 
stance, the space inthe middle of the triangle having Copernicus, Aristarchus, 
and Kepler at its angles (or more exactly between Milichius and Bessarion) is 
lower than the surface around Hortensius (between Copernicus and Kepler), 
but not so low as the Mare Imbrium, far away from the region of ray centres 
of which Copernicus, Aristarchus, and Kepler are the principal. 
