318 The Past History of our Moon. (July, 
One of the first and most obvious effects of this more 
rapid nuclear contraction would be the lowering of the level 
of the molten matter, which up to this period had been kept 
up to, or nearly up to, the lips of the great ringed craters. 
If the subsidence could place intermittently there would 
result a terracing of the interior of the ringed elevation, 
such as we see in many lunar craters. Nor would there be 
any uniformity of level in the several crater floors thus 
formed, since the fluid lava would not form parts of a single 
fluid mass (in which case, of course, the level of the fluid 
surface would be everywhere the same), but would belong to 
independent fluid masses. Indeed it may be noticed that 
the very nature of the case requires us to adopt this view, 
since no other will account for the variety of level observed 
in the different lunar crater-floors. If these ceased to be liquid 
at different times, the independence of the fluid masses is 
by that very fact established; and if they ceased to be liquid 
at the same time, they must have been independent, since, 
if communication had existed between them, they would 
have shown the uniformity of surface which the laws of 
hydrostatics require.* 
The next effect which would follow from the gradual re- 
treat of the nucleus from the crust (setting aside the with- 
drawal of lunar seas) would be the formation of corrugations, 
—in other words, of mountain-ranges. Mallet describes the 
formation of mountain-chains as belonging to the period 
when “the continually increasing thickness of the crust re- 
mained such that it was still as a whole flexible enough, or 
opposed sufficient resistance of crushing to admit of the 
uprise of mountain-chains by resolved tangential pressures.” 
Applying this to the case of the moon, I think it is clear 
that—with her much smaller orb and comparatively rapid 
rate of cooling—the era of the formation of mountain-chains 
would be a short one, and that these would therefore form a 
less important characteristic of her surface than of the 
earth’s. On the other hand, the period of volcanic activity 
which would follow that of chain-formation would be 
relatively long continued; for regarding this period as begin- 
ning when the thickness of the moon’s crust had become too 
great to admit of adjustment by corrugation, the compara- 
tively small pressure to which the whole mass of the moon 
had been subjected by lunar gravity, while it would on the 
one hand cause the period to have an earlier commencement 
* It is important to notice that we may derive from these considerations an 
argument as to the condition of the fluid matter now existing beneath the solid 
crust of the earth. 
