PLATE XXI. 



MARE NUBIUM AND SURROUNDINGS. PHOTOGRAPHED BY RITCHEY, NOVEMBER 21, 



1901, 7 HOURS 32 MINUTES P.M. EXPOSURE, ONE SECOND. 



SCALE, ONE-HALF METER TO MOON's DIAMETER. 



In this plate Copernicus is the large vulcanoid on the lower margin. The large crater near 

 the upper margin, a little to the right of the center, with a cone somewhat to the right of its center 

 and " rill " on its floor, is Pitatus. The three great vulcanoids in a row extending in a north and 

 south direction, are, in succession from the lowest towards the upper margin of the plate, Ptole- 

 mseus, Alphonsus, and Arzachel. The large deep crater below and to the right of Pitatus, with a 

 divided central cone, is Bullialdus. 



The most noteworthy features in this plate are found in the many instances in which the lavas 

 of the maria have partly destroyed the vulcanoids within their fields. In the upper right-hand 

 fourth of the plate, there are a dozen or more of these ruined craters, some of them with their walls 

 almost effaced. In this part of the field there are several important rills. Some of these are evi- 

 dently rows of craterlets in which the adjacent walls of the pits have been broken down so as to 

 form a ragged cleft. A number of these lines of craterlets are traceable on the external slopes of 

 Copernicus. The long, dark line, sixty-five miles in length, in the upper third of the plate, a little 

 to the left of the center, is the Straight Wall, the most extensive fault known on the moon. The 

 height of its cliff is about five hundred feet. The crescent shaped structure at its southern (upper) 

 end is the remnant of a crater, the remainder of the margin having been destroyed by the lava of 

 the mare. To the right of, and near by the Straight Wall, is a rill extending in a slightly curved 

 course for a length of about forty miles, terminating at either end in a distinct craterlet. 



The brightly illuminated part of the field depicted on this plate, that to the left of the center, 

 exhibits many excellent examples of crater valleys, which in their series afford something like a 

 passage from the condition of rills to those wider depressions. 



133 



