42 A COMPARISON- OF THE FEATURES OF THE EARTM AXP THE MOON. 



defined name of valleys. The most conspicuous depression ordinarily classed in 

 this group is the great Alpine valley which traverses the mountainous ranges of 

 that name, extending in a northeast direction from near the Mare Frigoris to the 

 Mare Imbrium, a distance of about eighty miles. (See plate xxni.) In width it 

 varies from four to six miles, but at its southern extremity for about one-fourth of 

 its length it is somewhat narrower, being reduced at one point to about two miles 

 in cross-section, and at the mouth it is beset with what seem to be extruded masses, 

 so that it debouches by several narrow clefts into the neighboring sea. The walls 

 of this valley are generally nearly vertical ; from my own comparisons with other 

 measured objects, they appear to average more than a mile in height and to be 

 for the greater part of their extent almost rectilinear. The floor of the depres- 

 sion is approximately level, though with some obscure pits, and by its color 

 as well as its form is evidently covered by an extension of the Mare Imbrium. 

 The Ukert valley, on the east side of the crater of that name, is longer than the 

 Alpine and has about the same width with less depth. The fracture by which it 

 was formed appears to be continued in an obscure cleft, which extends from its 

 northern end to the vicinity of the vulcanoid called Marco Polo, the whole con- 

 stituting what seems to be one structure nearly two hundred miles in length. A 

 similar valley with a length of about eighty miles lies on the west side of 

 Herschel. It has a width of at least ten miles and is rather straight-walled. 

 Yet another notable feature of this group is that lying on the eastern side of 

 Rheita, which is about one hundred miles in length and about twelve miles in 

 diameter. Last of all we may cite the great valley on the southwest side of 

 Reichenbach, which extends in a rather tortuous course for about one hundred 

 miles and has a width of ten or twelve miles. There are many other similar, 

 though smaller, valleys, varying from a maximum width of ten or twelve miles 

 downwards, until they grade in dimensions into the group of clefts. A full list 

 of these structures is lacking, but they probably number several score. 



As regards the distribution of the fault valleys, it is noteworthy that all the 

 distinct examples of the group lie outside of the maria. It is true that on those 

 fields there are depressions which have been classed with the vales, but, so far as I 

 have been able to determine, they all fail to exhibit the essential features of this 

 group—/, r., they lack the steep walls and the generally rather level floors char- 

 acteristic of the true valleys. They seem to my eye to be in their nature synclines, 

 or downward foldings, the counterparts of the continuous ridges which are so 

 characteristic of the maria, though they are not found in any definite relations to 

 those up-folds. As to the time of the formation of the valleys, it appears to have 

 been relatively late, posterior to the formation of the mountains, though before the 

 production of the lavas of the maria. It is not certain that any larger vulcanoids 

 than the craterlets were formed at a later stage in the evolution of the surface, 

 for only very small structures of this group appear to have been produced in their 

 cavities. It is also to be noted that these fault valleys are most developed in 

 the regions where the larger vulcanoids are not very abundant, though it must be 

 said that they are not lacking in the fields where these features are well developed. 



