34 A COMPARISON OF THE FEATURES OF THE EARTH A\I) THE MOON. 



have them compared with like structures in the lava floors of the larger ring 

 plains. My inspection shows them to be very similar in aspect, as they may be 

 in origin, probably being both alike due to actions taking place within a moderate 

 distance from the surface. 



MOUNTAINOUS RELIEFS OF THE MOON. 



Next in topographic importance to the vulcanoids come the reliefs, which 

 have received the general name of mountains. In this group we find at least 

 three distinct categories, which probably are due to as many separate causes. 

 First and most important of these species of salient forms come those which have 

 generally been named after terrestrial ranges or erogenic systems, as, for instance, 

 the Alps, Apennines, Caucasus, etc. Although these groups of elevations have 

 a considerable local diversity in character, varying in elevation from two or three 

 thousand to twenty-six thousand feet or more, and in shape of their individual 

 peaks from seldom nearly conical forms to much extended ridges, they in general 

 have the character of elongate masses rudely elliptical in horizontal section, the 

 several units of each field showing a tendency to a rude parallelism of their axes. 

 These units are rarely distinct from one another, but connected at their bases, so 

 that the field they occupy is by their confluence considerably raised above the 

 general surface of the country in which they lie. 



The number of these fields of mountains which have been named by sele- 

 nographers is about twenty-five. There are, however, probably at least twice as 

 many areas which exhibit this type of structure in a tolerably clear manner. One 

 of the most important of these is the area between Schroter on the south and 

 Marco Polo on the north, the area in part forming an isthmus-like barrier between 

 the Mare Nubium and the Sinus ^stuum. The facts go to show that while 

 the tendency to form this type of topography is more evident in the northern 

 than in the southern hemisphere, it has existed in some measure on all parts of 

 the moon except those now occupied by the maria ; in these fields, though there 

 appear to be ill-preserved remains of such structures, they are very imperfect. It 

 may also be said that structures of this nature seem to be more frequently 

 developed near the limb than elsewhere, but this may be due to errors in classi- 

 fication, consequent on the difficulty of determining whether elevations in that 

 part of the surface are the borders of vulcanoids or mountain ridges. 



In considering the relation of the mountains of the moon to the vulcanoids, 

 it is important first of all to note the fact that where they are extensively devel- 

 oped there is a prevailing absence of larger crater-form structures, and that in 

 certain instances we may at least suspect that they have broken up such struc- 

 tures. At a number of points involved in these tangles of ridges there are 

 features which look very much like fragments of the rampart of ringed plains 

 which had been involved in the apparently tumultuous movements attending the 

 building of the mountainous reliefs. Instances of this nature occur in nearly all 

 the larger mountainous areas ; good examples exist in the Haemus Mountains 



