48 A COMPARISON OF THE FEATURES OF THE EARTH AM) THE MUON. 



something like these diversities of action is to be seen in terrestrial lava fields, 

 though it is not certain that they are due to like causes. On any frozen expanse 

 of lava we are apt to find at once ridges which cannot well be attributed to the 

 ropins; of the solidified crust, along with cracks which are evidently due to super- 

 ficial cooling. There are other possible explanations of these contracted dislo- 

 cations of the maria, but I shall here take leave of the subject, for it is one on 

 which I have not been able to form a satisfactory opinion. 



ADJUSTMENTS OF THE SURFACE TO CONTRACTION. 



Looking over the whole of the lunar structures, the geologist is naturally 

 surprised to find so little in the way of adjustment of the crust of the sphere 

 to a nucleus diminished by the loss of heat. On the earth he sees in the ample 

 folds of the sea-basins and of the continents, as well as in very many folded 

 mountain chains, what he takes to be evidence of a long-continued accommodation 

 of an anciently cooled crust to a central mass which is ever losing heat. On the 

 moon he finds what, in proportion to the size of that sphere, is surely not the 

 hundredth part of such action. The folding of the marial ridges and furrows 

 is trifling and is probably due to action set up in the lavas of those fields. The 

 features of the crater valleys and the deformed vulcanoids appear to indicate 

 some small measure of folding, but that may have been brought about by the 

 loss of the moon's rotation through tidal action, and the consequent disappear- 

 ance of an equatorial bulging due to that rotation. In any event it does not 

 appear to represent any considerable readjustment of the crust to the interior. 

 It is true that the moon has only one-fourth the earth's diameter, and the fold- 

 ing caused by shrinkage should only be in about that ratio to like action on the 

 earth. Yet on the satellite the process of cooling is probably at an end, while 

 in the case of the earth, reckoning from the time when the crust was formed, 

 it cannot well be more than half accomplished. What then is the meaning of 

 this startling diversity in the orogenic history of the two spheres ? 



In considering the difficult problem which has been just above suggested, 

 the first question that comes before us is as to the value of the evidence concern- 

 ing the antiquity of the general surface of the moon. We may ask whether 

 the original sphere may not have cooled in its time to a low temperature, 

 making in the process the necessary adjustments of its outer crust to the dimin- 

 ished interior, and whether after that was all done the mass may not have been 

 added to by the in-falling of meteoric bodies, such as has been hypothesized to ac- 

 count for the maria. By such in-fallings a general outer coating of lava might 

 have been formed, only a few-score miles in thickness, and to this may be due all 

 the vulcanoid phenomena down to the time when the later coming of other such 

 bodies formed the maria. On the basis of this conjecture we would not have to 

 look for any extensive marks of readjustment of crust to central mass. It cannot 

 be denied that the body of any celestial sphere is liable to be added to by in-fall- 

 ing masses, at least until it has cleared its path of them ; and the fact that it has 



