THE MOONS FACE. 
283 
filled and Phocylides c is almost completely filled, though 
the largest, Phocylides, is empty. To the northwest of these 
and to the south of Mare Humorum is a broad tract char¬ 
acterized by much filling of craters and by the obliteration 
of minute craters. It has the general aspect of the Imbrian 
region, but I have not ventured to include it in the Imbrian 
chart, as it is separated by nearly 200 miles of the Nubian 
plain from the nearest district distinctly sculptured. If it 
did derive its overwash from the Mare Imbrium, then that 
a 
flood extended in this direction more than 1,500 miles, and 
must have swept over the entire Oceanus Procellarum. Be 
this as it may, Wargentin and the neighboring lowland 
probably have a common flood history. 
Rills and Rill Pits .—Among the most difficult of the 
moon’s enigmas is the problem of the “ rills.” Narrow 
defiles, often tapering at either end, they suggest fissures; 
but fissures taper downward also, and many of the rills 
have flat bottoms. Deep canyons, with parallel steep walls, 
they suggest stream beds; but stream beds have continuous 
descent in one direction, and the rills run up hill as well as 
down. 
Here again the Imbrian deluge affords a clue. Close by 
Julius Csesar, and in the same district of pasty—or, at least, 
non-liquid—overwash, lie the great rills of Ariadseus and 
Hyginus. The rounding of their edges marks them as 
antediluvial features; what was their condition before the 
flood? Certainly not the same as now, or they would have 
been filled and obliterated. I imagine them as yawning 
chasms three-fourths of a mile wide at top and several 
miles in depth. As the swift tide rushed over them a small 
portion may have been arrested and engulfed, but the 
chasms were not filled until the torrent stopped. Then 
that which spanned them sank down, coming to rest a short 
distance below the edges, and so forming the visible floors. 
The pits that interrupt these floors are definitely related to 
the rills and cannot be classed as impact craters. Possibly 
here and there an arch of debris that had clogged the 
