PLATE IX. 



Mare Nuhium and surroundings. Fhotixjraphed hy Ritchey, November 21, 1901, 7 hours 

 32 minutes p. m. Exposure, 1 second. 



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 some- 

 what 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 toward the upper margin of the plate, Ptolemauis, Alphonsus, and 

 Arzachel. The large, deep crater below and tf) the right of Pitatus, with a divide<l 

 central cone, is BuUialdus. 



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 f>f 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 evidently 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, 65 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 500 feet. The 

 crescent-shaped structure at its southern (upper) end is the remnant of a crater, the 

 remainder of the margin having been destroyed l)y 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 40 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 of wider 

 depressions. 



