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



considerable pit or a mountain in the center of the ring, the probability of this 

 central feature occurring being greater with the decrease of the size of the 

 vulcanoid, until the diameter of the plain becomes less than about ten miles, when 

 it tends to disappear. The facts indicate that the central pit and mountain of the 

 vulcanoid floor are interchangeable features. In some cases the peak has a more 

 or less distinct craterlet upon its summit, or, as is shown in the central compound 

 structure of Theophilus, there may be traces of a crater masked in the extruded 

 heap. 



The third group of reliefs on the lunar surface is typified by the long, low, 

 apparently continuous ridges which are found on all the maria, but which are 

 particularly well developed on the Mare Imbrium, the Mare Serenitatis, and the 

 Mare Nectaris. (See plates xvm and xxiv.) The characteristic features of 

 these ridges are their prevailingly low-arched forms, their slight height, and their 

 remarkable continuity ; they very often attain a length of one hundred miles, and 

 in some cases of twice or thrice that extent, while the greatest elevation assigned 

 to them is less than two thousand feet. As their flanks grade rather indistinctly 

 into the general surface of the maria, their precise width cannot be stated ; it is 

 evidently variable, with a probable maximum of five to ten miles. So far as I 

 have been able to ascertain, well developed continuous ridges are limited alto- 

 gether to the maria and practically so to the larger fields of this nature ; in the 

 small maria they are much less distinct, though there are instances of slight undu- 

 lations which may belong in the same category of structures. In fact all the 

 extended plains, even those of the greater vulcanoids, exhibit more or less 

 wrinkled surfaces, when seen with powerful telescopes under very oblique illumin- 

 ation, such as serves to bring out irregularities only a few score feet in height. 



The distribution of the continuous ridges indicates that they belong to two 

 distinct groups which may be due to diverse causes, or at least to different 

 methods of action of some general cause. The most evident of them are often 

 nearly rectilinear, or with broad curves, which have no evident relations to the 

 outlines of the shore of the mare in which they lie. Of these, the great examples 

 extending from near Lambert in the Mare Imbrium, or those of the Mare Sereni- 

 tatis lying between Posidonius and the promontory of Acherusia, may be taken 

 as types. Another group, well indicated on the borders of many of the maria 

 and some of their embayments, has the folds following the shores and seems to 

 be limited to a somewhat distinct field lying near those shore lines. Elger sug- 

 gests that in the case of Mare Nectaris these shore-following ridges are due to 

 the settlement of the lava in the central part of the basin. It is undoubtedly the 

 fact that the lava has been lowered in the Mare Crisium since the surface has 

 frozen, as it probably has in all the maria ; traces of like action seem to me to be 

 more than conjecturable in the floors of the larger vulcanoids as well ; but it is 

 not to me clear that these shore-following wrinkles are, as Elger suggests, caving- 

 in steps, such as those formed on the edges of a frozen pool or stream as the 

 water in the basin subsides. If they are, as some of my sketches indicate, 

 arranged in the manner of a carpet on a stairway, as monoclinal folds of terres- 



