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



these are the regions of so-called mountains, or in fields where there exist very 

 many low dome-like elevations, often circular in outline but occasionally some- 

 what elongate. Of those regions where vulcanoids of considerable size are rare, 

 the most noteworthy are the field of the Haemus Mountains, the region on the 

 west side of the Mare Fcecunditatis, and that to the northwest of the Caucasus 

 Mountains, though there are many others of about the same extent. (See plate 

 xvm.) Several of these regions are of more than fifteen thousand square miles 

 in area. It should be understood, however, that none of these fields entirely 

 lacks vulcanoids ; it is indeed doubtful if there is any part of the moon's surface, 

 except it may be some portions of the maria, where craters of large or small 

 size may not be found in every circle of twenty miles in diameter. 



In many accounts of the distribution of the lunar vulcanoids it is stated that 

 the greater of them exhibit a distinct train-like arrangement. As before noted, I 

 have been unable to find any satisfactory evidence of such order being at all 

 common. Here and there, as in the group of Ptolemseus, Alphonsus, and Arza- 

 chel, there is a trace of linear order, but a study of the facts shows that so far as 

 the larger structures are concerned there is no reason to believe that there is 

 any prevailing definite order in their placement. There is, however, good reason 

 to believe that the smaller vulcanoids, commonly termed craterlets, are not infre- 

 quently arranged in linear order. This is not true of all of them, but is clearly so 

 in the case of those which are in some way related to the rills or other crevices, 

 and to the light rays of this point I shall have more to say below. 



As regards the order of distribution in time of the lunar vulcanoids, it may be 

 said that all the facts point to the conclusion, if they do not establish it, that the 

 largest of them commonly were formed first. This is shown by the fact that 

 in only a few instances does a large ring plain cut a decidedly smaller structure of 

 the same nature, while the instances in which the smaller have intersected the larger 

 are very numerous. So far as I have been able to apply this method of determin- 

 ing the relative age of the rings, it establishes the fact that the greater number, if 

 not all, of the vulcanoids of say over fifty miles in diameter were completely formed 

 before the most, if not all, of those say twenty miles in diameter were built, and 

 further that very many of the craterlets were opened after the greater structures 

 were completed. Still further it appears likely, though not certain, that before 

 the greater vulcanoids were formed the so-called mountain districts and the 

 general surface of the moon had acquired the topography we now find them to 

 have, at least as regards the larger features of the surface. In very many of the 

 great vulcanoids we find evidence that the neighboring country has had its surface 

 somewhat distorted by the intruding structure. In a word, there appears to have 

 been an ancient surface antedating the distinct ring plains, though it is possible 

 that this surface was itself largely made up of such rings which have been obliter- 

 ated by the agents of decay, which have in many instances partly demolished 

 structures which are still recognizable, though often but faintly. The number of 

 these faint rings too indistinct to be named, and rarely affording more than the 

 merest traces of their original form, is so great as to warrant the conjecture that 



