A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 47 



earth, as the diversity in the development of this group of structures which they 

 exhibit. 



The next question is as to the group of lunar reliefs to which the continu- 

 ous ridges of the maria belong. It seems clear that, whatever be the detailed 

 structure of these ridges, they indicate compressive actions of the terrestrial 

 mountain-building type. The great linear extent of these compression ruptures 

 shows that they are due to no local strains but are the result of stresses which 

 pervaded wide fields of the maria. Their narrowness and lack of considerable 

 height may be taken as evidence that they are the result not of deep stresses 

 but of such as resided in the superficial parts of the crust, probably within the 

 lava of which these fields are composed. As to their age they of course post- 

 date the formation of the maria and apparently the larger vulcanoids none, 

 indeed, of great extent which have developed on their plains. It is obviously 

 important to determine the time of their uplifting in relation to that when the 

 rills were formed. This I have been unable to do in a satisfactory manner. I 

 have no notes of good examples in which either of these groups of structures are 

 found in intersection ; nor does my limited acquaintance with the literature of 

 the subject supply such instances. It appears, however, likely from the fresh 

 aspect of both groups of dislocations that they are not of very diverse age, but 

 that the rills are the newer. 



The problem presented to us is the existence in the same field of the rills 

 which indicate the shrinkage of the material of which the maria are composed, 

 together with that of the continuous ridges which even as clearly show that this 

 portion of the moon's surface has been in a state of compression that compelled 

 the rocks to buckle upwards and, if we have rightly interpreted the structures, 

 brought about the formation of corresponding synclinal forms, the shallow 

 troughs which exist on these plains. If it is proved, as seems likely 'to be 

 the case, that the rills on the maria were formed after the continuous ridges, 

 then we might conceive that the cooling of the interior of the moon brought 

 about a compressive strain on the already cold outer crust, and that the limited 

 diameter of those wrinkles was due to the fact that there was still some measure 

 of viscosity in the lower part of the lavas of the maria which made it possible 

 for the hard upper part of the sheet to act independently of the subjacent 

 portions of the section, so that this upper part of the sheet as a whole received 

 the compressive stress as a thrust from the shores against which it lay. 



There is another way in which we may consider this problem of associated 

 compression and shrinkage in the maria. It is to be noted that the most distinct 

 examples of each action lie in fields remote from each other, the rills near the 

 shores and the continuous ridges remote from them, the one in fields where the 

 lava is presumably rather shallow, the others where it is deep. With this 

 difference in conditions it might come about that contraction of the deeper parts 

 of the marial sheet in the process of cooling would be sufficiently strong to 

 fold the surface, while in the quickly-cooling shallow parts of the maria the 

 only effect would be the formation of shrinkage cracks. It is to be noted that 



