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



57 



of a tendency of the crust to split radially so as to permit the entrance of dike- 

 making lavas, these fractures are not known to extend for more than at most a 

 score of miles from the vent to which they center ; nor is there any observed 

 tendency of the crust about volcanoes to become penetrated with joint-planes, 

 having the position of those which the before-noted facts lead us to suppose 

 exist on the moon. Before this evident lack of likeness between the two spheres 

 is weighed, it is well to note that, while in the case of the earth all the extensive 

 jointing of the rocks is apparent, brought about by strains due to mountain- 

 building action, even when the beds have not been visibly dislocated it is evident 

 that they have been jointed by the stresses ; so that the fracture systems of the 

 earth may be said to depend on an action which does not appear to have been 

 to any considerable extent effective on the satellite. 



Given a sphere in which there are no extensive strains due to the contraction 

 of its central part and a consequent readjustment of the crust to the nucleus, 

 which appears to be the case with the moon, it is not unlikely that a series of 

 ruptures such as we find indicated by the rays would be formed. In such an 

 orb, the last stage of its cooling would necessarily lead to the contraction of its 

 outer part. Such was evidently the case in the moon, as is shown by the late 

 formation of its valleys and rills. After this strain had become so slight that it 

 was no longer competent to open distinct fissures, it might still have been sufificient 

 to produce the incipient tension cracks required for the escape of vapors such as 

 are needed to account for the light rays. 



The most difficult point to explain is the radial distribution of most of the 

 rays and the evident relation of nearly all of them to the greater vulcanoids or to 

 craterlets situated on their flanks. This, it seems to me, may be accounted for in 

 the way set forth below. Let us suppose that in the last stage of the expulsion 

 of the vapors of the lunar sphere, when the formation of vulcanoids of more than 

 about a mile in diameter was no longer possible, the cru.st was by its coolino- 

 brought into a state of contractile tension so that it had a tendency to break. 

 We may then fairly assume that this tendency would be greatest in the ancient 

 uplands, and least in the relatively new maria and in the lava tioors of the vulca- 

 noids. These fractures, or lines of weakness, for they do not seem to have been 

 defined openings of measurable extent, would naturally — indeed necessarily — 

 be made as radii to the large pits of the crust which plentifully occur in the 

 higher parts of the moon. We may have visible evidence of their necessity by 

 watching how shrinking clay splits in relation to holes made in its surface. 

 Beginning in the field about a vulcanoid, a fissure would extend radially for a 

 certain distance, far enough to satisfy the strain which led to its formation ; if it 

 afterwards happened, as W. H. Pickering has noted, that a body of vapor broke 

 its way to the surface, forming a craterlet at the point remotest from its origin, 

 then the rupture might be continued on the same line, attended by the formation 

 of another craterlet, until the strain was again satisfied ; and this process might 

 be again and again repeated until the greatest observed extension of the ray was 

 brought about. 



