1874.] The Past History of our Moon. Bhs 
Now let us conceive the somewhat thickened crust con- 
tracting upon the partially fluid nucleus. If the crust were 
tolerably uniform in strength and thickness we should expect 
to find it yielding (when forced to yield) at many points, dis- 
tributed somewhat uniformly over its extent. But this 
would not be the case if—as we might for many reasons 
expect—the crust were wanting in uniformity. There would 
be regions where the crust would be more plastic, and so 
readier to yield to the tangential tensions. Towards such 
portions of the crust the liquid matter within would tend, 
because there alone would room exist for it. The down- 
drawing, or rather in-drawing, crust elsewhere would force 
away the liquid matter beneath, towards such regions of less 
resistance, which would thus remain at (and be partly forced 
to) ahigherlevel. At length, however, the increasing tensions 
thus resulting would have their natural effect; the crust 
would break open at the middle of the raised region, and in 
radiating rifts, and the molten matter would find vent through 
the rifts as well as at the central opening. The matter so 
extruded, being liquid, would spread, so that—though the 
radiating nature of the rifts would still be indicated by the 
position of the extruded matter—there would be no abrupt 
changes of level. It is clear, also, that so soon as the out- 
let had been formed the long and slowly sloping sides of the 
region of elevation would gradually sink, pressing the liquid 
matter below towards the centre of outlet, whence it would 
continue to pour out so long as this process of contraction 
continued. All round the borders of the aperture the crust 
would be melted, and would continue plastic long after the 
matter which had filled the fissures and flowed out through 
them had solidified. Thus there would be formed a wide 
circular orifice, which would from the beginning be consider- 
ably above the mean level of the moon’s surface, because of 
the manner in which the liquid matter within had been 
gathered there by the pressure of the surrounding slopes.* 
* T have occasion to make some remarks at this stage to avoid possible and 
(my experience has shown me) not altogether improbable misconception, or 
even misrepresentation. The theory enunciated above will be regarded by 
some, who may haye read a certain review of my Treatise on the Moon, as 
totally different from what I have advocated in that work, and, furthermore, as 
a theory which I have borrowed from the aforesaid review. I should not be 
particularly concerned if I had occasion to modify views I had formerly ex- 
pressed, since I apprehend that every active student of science should hope, 
rather than dread, that as his work proceeds he would form new opinions. 
And again, Iam not in the least anxious to claim priority as to the enunciation 
of any theory, conceiving that claims of the kind seem as a rule indicative of 
a singular poverty of intellect on the part of those who make them (as though, 
having given birth to one good thought, they had no hope of ever being 
