Mare Vaporum and the Lunar Clefts. 57 



elastic force burrowing " straight on end " like a mole under 

 the ground, and, nine times out of ten, finding less resist- 

 ance in its front than in any other direction ; while, if such a 

 ridge were subsequently to " cave," or drop in, it would leave 

 no noticeable hollow. It seems more consistent with appear- 

 ances to refer these clefts to the cracking and splitting, which 

 would result from a contraction affecting the superficial more 

 rapidly than the subjacent material. Such an explanation would 

 not be without serious difficulties, from the very plastic con- 

 dition of the surface which would be required, and the incon- 

 sistency of such a state with the sharpness and cleanness of 

 earlier outbursts ; still it may claim attention ; but we are as 

 yet too little advanced in the collection and comparison of facts 

 to be able to form any very conclusive generalization. One 

 thing is evident ; that if we follow the example of Schmidt in 

 including the Great Valley of the Alps in the catalogue, it is 

 there that we shall find the most satisfactory test of our hypo- 

 theses, and that nothing can be admitted which fails to account 

 for the production of that colossal gorge. 



At any rate, B. and M. consider it certain that with, 

 perhaps, a few exceptions, these clefts belong to the latest 

 epoch of the lunar formations. Assuming the identity of their 

 origin with that of the long low ridges, we may remark that it 

 does not appear how this idea can be made to harmonize with 

 the aspect of these latter, which in so many cases look like 

 channels of communication contemporaneous with the forma- 

 tion of the more important mountains and craters ; but there 

 seem to be instances in which the posterior date of the clefts is 

 capable of proof; and one such will shortly be laid before us. 

 Considering it as well established that their general epoch is 

 comparatively a modern one, these observers do not wholly 

 deny the possibility of such processes being still in opera- 

 tion; and they admit that their opinion to the contrary, 

 however strong, rests only upon the experience of seven or 

 eight years. 



The inquiry — a very curious one — has also been treated as an 

 open question by a modern authority of great weight, Dr. 

 J. F. J. Schmidt. This eminent astronomer has informed us, 

 in his interesting memoir on the " Rills " of the Moon, that he 

 has been occupied for twenty-five years in the study of lunar 

 topography, during which he has made upwards of 1000 draw- 

 ings and 3000 height-measures, without coming to any other 

 conclusion than that no change of surface has recently taken 

 place on any considerable scale ; but that, as to the progres- 

 sive multiplication of these minute clefts, he has by no means 

 abandoned the idea of its possibility. He has succeeded in 

 adding a great number to those already known ; but this may 



