A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 21 



due to the melting down of the borders of other craters as its area was extended 

 in the manner which we shall hereafter see to have been common in the develop- 

 ment of the larger crater-form pits. In other instances, as in Ptolemreus, the 

 irregularity of the crater's shape may lead to doubt as to its classification, yet it 

 is regarded by Elger as one of the most characteristic walled plains, its rampart 

 being exceptionally good. A further analysis of the instances which at first 

 sight appear to lead to some doubt as to the existence of a sharp line parting 

 the maria from the vulcanoid floor leads to the same conclusion as the facts 

 previously set forth, that these groups of level areas are, as structures, completely 

 separated from one another, and therefore cannot have had like histories. In 

 the one there has been a long-continued local volcanic-like action leading to the 

 formation of an external rampart ; in the other, a swift production of an igneous 

 fluid, which has swept away until it found its level and shaped its margin by 

 melting down the pre-existing reliefs. 



Although in general the material which forms the floors of the several 

 maria appears to be confluent, /. e., to show no marks of overlapping at the lines 

 of junction, there is reason to believe in the opinion of many observers that 

 there is some diversity in the level of their floors. Thus the Mare Nectaris is 

 supposed to be decidedly deeper than the others. This is not inconsistent with 

 the view that they were all formed at nearly the same time. The greater depth 

 of the last-named mare may be explained by the supposition that the absorption 

 of the fluid matter into the ancient crust was relatively greater there than 

 elsewhere. 



While the surfaces of the maria are, as compared with the general surface 

 of the moon, decidedly plain-like, they are, in fact, the seat of many irregularities. 

 Of these the more important are a multitude of more or less continuous low- 

 arched ridges, probably in no instance more than two thousand feet high, but uni- 

 formly of relatively great width, often several miles in transverse section. The 

 nature of these ridgres will be hereafter discussed. There are also on the maria 

 numerous craters, none of them approaching in magnitude those on the old, more 

 elevated portions of the crust. The ratio of craters on the maria is only about 

 one-fifth as great as on equal areas of the original surface, and their average 

 size is in about the same proportion. It is also to be noted that rifts or open 

 cracks are apparently rarer on the maria than on the high lands and that the 

 light bands and patches are of relatively seldom occurrence. 



CLASSIFICATION OF VULCANOIDS. 



In considering the so-called volcanoes of the moon (I shall term them vul- 

 canoids), the first step should be a classification of their features. Selenologists 

 have generally agreed to distribute them in seven categories termed as follows : 

 walled plains, mountain rings, ring plains, craters, crater-cones, craterlets, crater 

 pits. Besides these groups they recognize the existence of a less characteristic 

 group to which they give the ill-defined name of depressions. Under the term 



