110 GENEEAL DESCRIPTION OF THE MOON. 



some way, and rather nearly, related to the volcanic vents of the earth 

 appear.s certahi. We have now to note the following peculiar condi- 

 tions of these pits. First, that they exist in var3'ing proportion, with 

 no evident law of distribution, all over the visible area of the moon. 

 Next, that in many instances they intersect each other, showing that 

 they were not all formed at the same time, but in succession; that the 

 larger of them are not fomid on the maria, ]>ut on the upland and 

 apparently the older parts of the surface; and that the evidence from 

 the intersections clearly shows that the greater of these structures are 

 prevailingly the elder and that in general the smallest were the latest 

 formed. In other words, whatever was the nature of the action 

 involved in the production of these curious structures, its energy 

 diminished with time, until in the end it could no longer break the 

 crust. 



All over the surface of the moon, outside of the maria, in the regions 

 not occupied by the volcano-like structures, we find an exceedingly 

 irregular surface, consisting usually of rude excrescences with no dis- 

 tinct arrangement, which may attain the height of many thousand feet. 

 These, when large, have been termed mountains, though they are 

 very unlike any on the earth in their lack of the features due to 

 erosion, as well as in the general absence of order in their association. 

 Elevations of this steep, lumpy form are common on all parts of the 

 moon. Outside of the maria they are seen at their best in the region 

 near the north pole, where a large field thus l)eset is termed the Alps. 

 From the largest of these elevations a series of like forms can be made 

 of smaller and smaller size until they become too minute to be revealed 

 by the teles(?ope; as they decrease in height they tend to become more 

 regular in shape, ver}^ often taking on a dome-like aspect. The only 

 terrestrial elevations at all resembling these lunar reliefs are certain 

 rarely occurring masses of trachytic lava, which appear to have been 

 spewed out through crevices in a semifluid state, and to have been so 

 rapidl}^ hardened in cooling that the slopes of the solidified rock 

 remained very steep. The only reliefs on the moon that remind the 

 geologist of true mountains are certain low ridges on the surfaces of 

 the maria. 



The surface of the moon exhibits a very great number of fissures or 

 rents which, when widely opened, are termed valleys, and when nar- 

 row, rills. Both these names were given because these grooves were 

 supposed to have been the result of erosion due to flowing water. 

 The valleys are frequently broad, in the case of that known as the 

 "Alpine Valley," at certain places several miles in width; they are steep 

 walled, and sometimes a mile or more in depth; their bottoms, when 

 distinctly visible, are seen to be beset wnth crater-like pits, and show 

 in no instance a trace of water work, which necessarily excavates 



