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



reader that these features are volcanoes. That view of their nature was taken 

 by the astronomers who first saw them with the telescope and has been generally 

 held by their successors. That they are in some way, and rather nearly, related 

 to the volcanic vents of the earth appears certain. The nature of this relation is 

 discussed below. We have now to note the following peculiar conditions of 

 these pits. First, that they exist in varying 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 found on the maria but 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 pre- 

 vailingly 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 distinct arrangement, which rhay 

 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 associa- 

 tion. 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 beset 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 telescope ; as they decrease 

 in height they tend to become more regular in shape, very 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 semi-fluid state, and to have been so 

 rapidly hardened in cooling that the slopes of the solidified rock remained very 

 steep. As noted in more detail below, the only reliefs on the moon's surface 

 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 open are termed valleys, and when narrow, 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 with crater-like pits, and show in no in- 

 stance a trace of water work which necessarily excavates smooth descending 

 floors such as we find in terrestrial valleys. The rills are narrow crevices, often 

 so narrow that their bottoms cannot be seen ; they frequently branch and in some 



