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



Supposing rays be formed by successive developments and reliefs of cooling 

 strains in the manner just above suggested, we find a reason for the peculiar 

 shape of these features which Pickering has well observed. He finds that the 

 longest of them are not strictly continuous, but that originating at a craterlet they 

 extend for a variable distance, widening and becoming dimmer the farther from 

 the place of origin ; then at another craterlet they again begin narrow and 

 bright, to fade and widen once more as they become remote from the opening. 

 According to my view, the first craterlet started the fracture ; near it the fissure 

 was most passable to the vapors in question, so there the streak is narrow and 

 bright ; farther away the fissure was less open, so that the effusion had to force 

 its way through the country rock, and so made a wider and fainter deposit of the 

 shining material. The second craterlet developed an extension of the fracture 

 with the same features as the first, and so on to the end of the colored belt. 

 According to this hypothesis, we need not suppose any such mighty accident as 

 required by the view that the ray system of Tycho was formed at once ; it may 

 have been geologic ages in developing ; the end of a great ray may, indeed, have 

 been formed very long after its beginning. 



To those who are unfamiliar with the movements of homogeneous materials 

 in the process of shrinking, it may seem unlikely that the outer part of the moon 

 in cooling equally would tend to fracture in systems of joints arranged in radial 

 order. A little observation on drying clay will show that slight accidents 

 determine in very uniform materials the direction of the fractures due to strains 

 which lead to cracking. When the pull is equal in every direction and when 

 there are depressions on the surface, the tendency is to make these pits the 

 center of radiating fractures. In this way, by cracks running from many centers, 

 the general tendency to rupture is satisfied. On the visible surface of the moon 

 there are near two-score recognizable ray systems, differing much in the distinct- 

 ness and extent of their light streaks. As these systems are widely scattered, 

 they are perhaps sufficient to have satisfied all the shrinkage strains of the crust 

 during the time when there were still vapors seeking to pass to the surface. 



As to the age when the rays were formed, it appears evident that they 

 were not all made at or near the same time. Those of certain systems ap- 

 pear to cut those of other systems. Thus, according to Nicoll as quoted by 

 R. A. Proctor, the rays of Copernicus, Aristarchus, and Kepler cut one 

 another in an order indicating that they were formed in the succession in which 

 they are here named. It also appears possible that the greater part of the 

 ray systems were formed before the maria were produced, for relatively but few 

 extend over their fields, though it may be that their general failure to traverse 

 these bodies of lava and also those contained in the craters of the greater vulca- 

 noids is due to some condition of the material which diminishes the shrinkage 

 tension existing on other and older parts of the moon. That the light "rays ante- 

 date certain of the rills, and perhaps all of them, is shown by the fact that they 

 are cut by these fissures. It should, however, be noted that Trouvelot, who had 

 a very keen eye, noted that certain of these open crevices are continued beyond 



