20 A COMPARISON OF THE ?'EATURES OF THE EARTH AND THE MOON. 



warranted as a working hypothesis, though it has, perhaps, not been established 

 as a theory. 



To the sueo'estion that the surface of the maria is in general lower than 

 that of the reeions surroundintr them, and that this fact is inconsistent with the 

 addition to the quantity of matter in the area they occupy, such as would be 

 brought about by the falling in of a bolide, the following answer may be made. 

 In the first place, it is to be noted that the outer part of the moon is, except in 

 the maria and in the crater floors, evidently characterized by a very open struct- 

 ure. It is prevailingly much occupied by volcanic openings, greatly rifted and 

 probably composed of scoriaceous materials. If any such section as that about 

 the Apennines were completely fused to the depth of some miles, it is likely that 

 we would have a subsidence of the surface quite as great as that exhibited by 

 the maria. In the second place, the bulk of the material brought by the bolide to 

 the lunar surface would be small as compared with the volume of matter which 

 would be melted by its impact. The proportion would probably be less than one 

 to ten ; so that the contribution from the impinging body would be so small 

 that it would not be likely much to affect the general level of the melted area. 

 The nature of the lunar surface in the maria and on the other more extensive 

 reeions will be further considered in the section on volcanic action. 



As before noted, there is no series connecting the ordinary craters, however 

 large they may be, with the maria. That this is the case is well indicated by the 

 fact that selenographers have in only a few instances been in doubt into which 

 group individual examples of these two species of lunar forms should be placed. 

 The fields classed as seas, with the evidently related embayments thereof, termed 

 sinuses or paludines, have always been regarded as readily distinguishable from 

 the craters. This decision has not been made on the basis of well-described 

 categories, but on the immediately evident differences between the two groups 

 of forms. It is recognized that while nearly all the vulcanoids are essentially 

 circular, or with only moderate distortions of that outline, the seas are as gen- 

 erally irregular in outline. So, too, it is patent that the vulcanoids, at least 

 those of large size, have in all cases a fairly well-marked external slope or cone. 

 None of the seas are thus characterized except where their periphery in part 

 corresponds to some antecedent feature, such as the wall of a large pit which 

 they have invaded, as in the case of Fracastorius, on the margin of the Mare 

 Humorum, or where it encounters an elevation such as the Haemus Mountains, 

 on the southern border of the Mare Nectano. (See plate xxv. ) This gen- 

 eral acceptance of an essential difference between the vulcanoid floors and the 

 seas, and the very slight doubt as to the classification of the level surfaces in one 

 or the other, is excellent evidence as to their difference in nature. 



The only areas of a level surface on the moon which may not be on mere 

 inspection classed as maria or vulcanoid floors are a few large crater-form de- 

 pressions situated near the eastern limb of the moon, of which the most import- 

 ant and doubtful is Schickard. Even a slight examination of this feature shows 

 that it has a distinct continuous wall, and that the irregularities of its outline are 



