46 A COMPARISON OF THE FKATURES OF THE EARTH AND THE MOON. 



It appears, however, most probable that they are due to the erogenic strains 

 which enter into the complex of actions involved in mountain building, combined 

 with some withdrawal of support ordinarily afforded by the materials of the under 

 earth, as would be brought about by the migration of matter seeking volcanic 

 vents. In the simpler and more applicable case of these down-faulted blocks of 

 the crust, such as occasionally occur about terrestrial volcanoes, we may fairly 

 assume that the sinking was due to the ejections which had made the under earth 

 unable to support the load. That such deficiencies of support would have locally 

 resulted from the lunar eruptions is highly probable. To this action then, with 

 fair probability of its truth, we may for the present refer the valleys of the Alpine 

 type. The minor cleft valleys radiating from the vulcanoids are evidently to be 

 most reasonably explained on the same hypothesis. They are, indeed, so far as 

 I can see, comparable to the \'al del Bove of j'Etna. 



The rills, where we have relatively narrow crevices, which seem to extend 

 indefinitely downward, with no distinct floors, may be regarded as due to the 

 secular refrigeration of the superficial parts of the lunar sphere at a time so late 

 that they found their way to no bodies of lava. They are evidently contraction 

 cracks formed on a very extensive scale. Where they are limited, as is often the 

 case with the smaller of them, to the lava field of a large vulcanoid, they may 

 represent no more than the contraction of that body of lava. When, however, 

 they are on the maria, an indefinitely extended sheet of the frozen material may 

 find relief in the fracture. The predominance of the greater rills on and about 

 the maria may be due to the fact that, whatever was the origin of those vast 

 bodies of once igneously fluid rock, the consequence of their appearance on the 

 moon's surface was, when they cooled, a great necessity for contraction. Not 

 only were the lavas of the maria originally at a high temperature, but they must 

 have communicated this heat to their shores and to the high country near them, 

 with the result that new and extensive readjustments due to cooling would be 

 required in those portions of the crust which had been thus affected. Thus the 

 rills and the Alpine valleys appear to be distinctly diverse in origin, the former 

 being due to loss of temperature of the crust in general, the latter to more com- 

 plicated action. 



As regards the rare instances of true displacement faults such as the Strait 

 Wall, they appear to be due to ordinary faulting such as so abundantly occurs 

 on the earth. They may in their first stage have been rills where there was 

 some lack of support which caused the rocks on one side of the fracture to 

 change their level with reference to those on the other. The only peculiar 

 feature about them, from the point of view of geology, is that they are so rare 

 and apparently so unconnected with compressive strains. If the surface of the 

 earth as it has been affected by faulting, but without the effects of erosion, 

 could be examined under the conditions in which we behold the moon, the fault 

 dislocations would appear by the hundred thousand and with vertical displace- 

 ments of miles in height. Nothing, indeed, so well illustrates the very great 

 difference in the history of these two neighboring spheres, the moon and the 



