A COMPARISON OF THE FEATURES OF THE EARTH AND THE MOON. 41 



which may be allied to those above described ; of these perhaps the most inter- 

 esting is that which forms the border of the Sinus Iridum and of the Mare 

 Imbrium, to the northwest and southeast of that remarkable bay; nearly the 

 whole eastern shore of the Sinus Roris and of the Oceanus Procellarum may be 

 of this nature. Though the last-named escarpment does not rise suddenly to 

 any great height above the mare plain, the straightness of the line suggests that 

 it was originally of greater vertical extent and was formed by faulting. 



The principal objection to the hypothesis above stated, that the above-de- 

 scribed features are due to faulting, is found in the fact that clear instances of such 

 action are rare on the lunar surface. The most conspicuous fault, where there can 

 be no doubt as to the nature of the conditions, is that commonly known as the 

 Strait Wall on the surface of the mare between Birt and Thibet. (See plate xxi.) 

 Here the break has a length of at least sixty-five miles and is quite as rectilinear 

 as any terrestrial fault. The vertical dislocation is at least five hundred feet and 

 may be much greater. It is evident that this is relatively a modern feature, having 

 been formed after the time when the mare had cooled. It is not unlikely that in 

 the earlier ages, when the moon was parting more freely with its heat, the re- 

 sulting faults were of far greater extent than is shown in the Strait Wall. It is 

 to be noted that the break of the Strait Wall did not lead to the extrusion of any 

 considerable amount of igneous matter. Elger has observed craterlets and 

 mounds upon the crest of the escarpment, but it is not clear that these are 

 genetically connected with the break, for such features abound in the Mare 

 Nubium as in other seas. Thus, though there is no basis for certainty, I am 

 disposed to regard the Altai group of escarpments as due to faulting. 



As to the age of the great escarpments above described, it may be said that 

 they certainly antedate the maria, which have their margins to some extent 

 determined by them. They seem also to antedate some of the larger vulcanoids, 

 for Piccolomini, which is about sixty miles in diameter, being in size among the 

 score of greatest structures, was formed after the Altai escarpment. Plato also, 

 though less clearly, appears to have been formed after the steep which bounded 

 the Alps on the south, now somewhat effaced by the Mare Imbrium, was devel- 

 oped. If this hypothesis, which seeks to account for the steep faces of highlands 

 by faulting, be correct, we must regard these features as among the most ancient, 

 perhaps the very oldest, reliefs on the lunar surface. They are now to a great 

 extent masked by the maria, which have found in them their natural shores, they 

 being, it would appear, bordered by them for near half their total coast line. 

 Further reference to these features will be made in the discussion of erogenic 

 action. 



VALLEYS, RIFTS, AND RILLS. 



In addition to the above-described positive reliefs of the moon, the surface 

 of that body presents a multitude of minor depressions which demand considera- 

 tion ; of these the most notable are the cavities which have received the obscurely 



