A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 45 



As regards the number of the rill fissures on the visible part of the moon we 

 have no good evidence. They are probably to be numbered by thousands, and 

 as the fainter seem to be the more plentiful, more effective instruments may 

 reveal many thousands of them. As regards their distribution there are many 

 noteworthy features. First we observe that those which have been mapped show 

 an obvious tendency to be arranged in groups, and in these groups the individual 

 breaks show here and there a tendency to intersect one another, though they are 

 more often arranged in a parallel relation. The next point is that those which 

 are in appearance sufficiently conspicuous to be mapped lie mostly in the cen- 

 tral part of the visible surface between the parallels of 30° north and south of 

 the moon's equator, and within 30° east and 50° west of the central meridian. 

 They are thus remarkably rare in high latitudes and apparently seldom near the 

 east and west margins of the visible part of the sphere. This apparent feature 

 of distribution may be due to the oblique view of those marginal fields. It is 

 also to be noted that all the important fractures are situated on or near the maria, 

 or on the floors of the greater vulcanoids. Of about seventeen examples mapped 

 by Elger, twelve intersect the shores of maria, and none of them lies altogether 

 more than one hundred miles from those lines. The great southern upland has 

 no mapped examples and the central parts of the larger maria are also without 

 them. I am aware that the floors of the greater vulcanoids abound in rills all of 

 small size. 1 am also aware of the fact that somewhere about a thousand of 

 these rill fractures have already been noted and that their distribution is much 

 wider than that indicated where only the more important are plotted, yet there 

 is probably some significance in the grouping of these greater specimens of the 

 class in or near the maria. 



As to the time of the formation of the rills, it may confidently be said that 

 they appear to be, with the possible exception of some of the craterlets, the most 

 recent structural features of the moon. If narrower scrutiny than has yet been 

 given to the matter shows that craterlets have developed in the cracks, then the 

 later structures, of course, postdate the rills. If, however, as it seems to me 

 quite possible, the rills have merely followed lines of incipient fracture, such as 

 joint planes would afford, in some instances going around the pits instead of 

 cutting through them, the rills may be the very last of the considerable lunar 

 accidents. Such, indeed, they seem to me to be. 



OROGENIC ACTION. CAUSES OF DISLOCATIONS. 



We turn now to consider the possible causes of the dislocations on the lunar 

 surface which are represented by the various kinds of valleys, clefts, rills, and 

 ridges which have been briefly described above. First, as to the valleys of the 

 Alpine type, it may be said that they appear to correspond to the Grabcn type of 

 terrestrial down-faultings, where there are two or more approximately parallel 

 faults, the included area having been lowered. As to the origin of geological 

 Graben, we have as yet no evidence of value and naturally no consensus of opinion. 



