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



CLEFTS AND RILLS. 



The clefts and rills of the lunar surface are features which seem to me 

 to belong in one group, though they may reasonably be separated from one 

 another by certain differences. Among the clefts we may class the very numer- 

 ous rifts which intersect the walls of the vulcanoids, particularly those of larger 

 size, which often extend for considerable distances beyond the limits of the 

 ramparts in which they occur. In the characteristic examples of this group, the 

 features radiate from the crater, and are thus shown to be in some way connected 

 with its conditions. They closely correspond in appearance with the Val del 

 Bove on the eastern versant of .^Etna and many like structures on other terrestrial 

 volcanoes. In some cases they appear to be essentially akin to the terrestrial 

 Graben or multiple fault depressions, as for instance the Alpine valley, in that the 

 ground between two fractures has been lowered. They may, indeed, be regarded 

 as a variety of that class of depressions determined by the strains originating in a 

 vulcanoid. There are very many examples of the group, ranging from those 

 which produce broad breaches in the crater walls to such as are shown on the 

 flank of Tycho, where the two parallel light streaks, which appear to follow the 

 path of faults, have the ground between them apparently somewhat lowered, 

 in the manner of a rather gentle syncline, without any evident displacement. 



Related to the several fault groups of depressions in that they are alike the 

 results of fracturing of the crust are the remarkable features known as rills. In 

 this group we have a single fracture with a space separating the walls, but no 

 distinct indications of a floor between them. Perhaps the most characteristic 

 example of the group is that known as the Sirsalis Rill, so named because the 

 Sirsalis vulcanoid lies near to it. Elger's description of this structure he evi- 

 dently knows it well is as follows : " Commencing at a minute crater on the north 

 of it [= Sirsalis], it grazes the foot of the Glacis, then passing a pair of small 

 overlapping craters (resembling Sirsalis and its companion in miniature), it runs 

 through a very rugged country to a ring plain east of De Vico [De Vico a] which 

 it traverses, and still following a southerly course, extends toward Byrgius, in the 

 neighborhood of which it is apparently lost at a ridge, thougji Schmidt and 

 Gandilot have traced it still farther in the same direction. It is at least three 

 hundred miles in length and varies much in width and character, consisting in 

 places of distinct crater rows." It has been suggested, according to Elger, who 

 does not state by whom, that the rills are not in fact breaks but a series of small 

 craters so near to one another that the effect on the eye is that of a continuous 

 crevice. This view, according to my observations with the excellent fifteen-inch 

 Mertz refractor of Harvard University, is not maintainable ; while craterlets are 

 often present along the line of the rill, their nature as fractures, when clearly 

 seen, appears certain. The breaks are ragged, as if torn through a row of crater- 

 lets, not usually more than half a mile in diameter and often narrowing at one 

 or both ends, so that their terminations cannot be determined ; but that they are 

 in their essence rents seems to me beyond doubt. 



