38 Condition of the Moon's Surface. [January, 



generated by the downfall a vast circular region of the 

 moon's surface would be rendered liquid, and that in rapidly 

 solidifying while still traversed by the ring-waves resulting 

 from the downfall, something like the present condition 

 would result. Or we might suppose that the region liquefied 

 through the effects of the shock was very much larger than 

 the meteoric mass ; and that while a wave of disturbance 

 travelled outwards from the place of impact to be solidified 

 (owing to rapid radiation of heat) even as it travelled, a 

 portion of the liquid interior of the moon forced its way 

 through the opening formed by the falling mass. But such 

 ideas as these require to be supported by much stronger 

 evidence than we possess before they can be regarded as 

 acceptable. I would remark, however, that nothing hitherto 

 advanced has explained at all satisfactorily the structure of 

 the great crateriform mountain ranges on the moon. The 

 theory that there were once great lakes seems open to diffi- 

 culties at least as grave as the one I have just considered, 

 and to this further objection, that it affords no explanation 

 of the circular shape of these lunar regions. On the other 

 hand, Sir John Herschel's account of the appearance of these 

 craters is not supported by any reasoning based on our 

 knowledge of the actual circumstances under which vol- 

 canic action proceeds in the case of our own earth. "The 

 generality of the lunar mountains," he says, " present a 

 striking uniformity and singularity of aspect. They are 

 wonderfully numerous, occupying by far the larger portion 

 of the surface, and almost universally of an exact circular 

 or cup-shaped form, foreshortened, however, into ellipses 

 towards the limb ; but the larger have for the most part flat 

 bottoms within, from which rises centrally a small, steep, 

 conical hill. They offer, in short, in its highest perfection 

 the true volcanic character, as it may be seen in the crater 

 of Vesuvius ; and, in some of the principal ones, decisive 

 marks of volcanic stratification, arising from successive de- 

 posits of ejected matter, may be clearly traced with power- 

 ful telescopes. What is, moreover, extremely singular in 

 the geology of the moon is, that although nothing having 

 the character of seas can be traced (for the dusty spots 

 which are commonly called seas, when closely examined, 

 present appearances incompatible with the supposition of 

 deep water), yet there are large regions perfectly level, and 

 apparently of a decided alluvial character ?" 



It is obvious that in this description we have, besides 

 those features of volcanic action which might perhaps be 

 •expected on the moon, a reference to features essentially 



