A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 55 



emitted gases required to produce a sufficient current even in a vacuum would 

 have to be impossibly great to make such a wind at the distance of hundreds (in 

 one instance 1 700) of miles from the vent. Moreover, as W. H. Pickering has 

 well shown, these bands do not, in all cases, point to any large crater, but in the 

 case of the most remarkable group that of Tycho appear to originate not in 

 the main vent but in certain small craterlets somewhat on one side of that open- 

 ing. Moreover, as well observed by Pickering, these bands are not definitely 

 continuous but made up of relatively short strips of bright-colored surface, each 

 of which appears to originate in a craterlet and to fade as it extends to another 

 in the same line, and that this arrangement probably continues to the end of 

 each streak. 



It is also to be noted that in some instances the bright rays of the moon show 

 a tendency to be parallel, or approximately so, to one another, they being in some 

 way causally related to rows of small vulcanoids. I have already called attention to 

 the existence of such near approach to parallelisms in the case of the two striking 

 examples in the Tycho system. There is another equally good example in the case 

 of Messier, where the two streaks of this system, though slightly divergent, show 

 an evident departure from the normal radial order. Many other instances could 

 be cited to show that, while these bands of lighter color obviously tend to be 

 placed in radial position with reference to a vulcanoid, they are here and there 

 affected by some conditions which warp them from that position and force them 

 to become parallel. This later condition is much more common in the numerous 

 faint streaks which cannot be referred to any group radiating from a large vul- 

 canoid. To my eye, this tendency to parallelism affects a considerable part of 

 the rays which appear to be of the older origin. 



It is obviously important to 'determine whether the rays of bright color on 

 the lunar surface are due to superficial conditions alone, or whether they are the 

 result of some action affecting the crust beneath the surface. On this point we 

 have little information but that of a highly indicative kind. A glance at these 

 features when they are best presented shows the observer that they extend 

 across the irregularities of the broken country they traverse. In at least one 

 instance, a ray emanating from the Tycho center crosses the lava plain in the 

 bottom of another crater (Saussure) and apparently traverses the steep slopes 

 of its wall, while another ray of this group seems to have been deflected from its 

 normal course by the ramparts of this vulcanoid. I have personally verified the 

 observations on the passage of this streak over the lava plain of Saussure and 

 have, though imperfectly, traced its passage up the inner wall of the rampart. 

 Other more skilled observers appear to have no doubt that it exists. 



The facts just above noted make it evident that the light rays are not 

 purely superficial features, but are in some way connected with the structure of 

 the crust ; from the point of view of the geologist, they have to be accounted for 

 by supposing that they are the superficial expression of an action essentially 

 solfataric in its nature, wherein vapors of some crystallizable substance, or sub- 

 stances, have passed through crevices of joint-like nature from the deeper parts 



