244 
GILBERT. 
above, this ridge calls to mind a wreath, and it has been so 
named. From the outer edge of the wreath a gentle slope 
descends in all directions to the general surface of the moorn 
which it is convenient to call here the outer plain. The 
outer slope of the crater may be identical in surface char¬ 
acter with the outer plain, or it may be radially and some¬ 
what delicately ridged, as though by streams of lava. The 
inner slope* from the base of the cliff to the margin of the 
Fig. 2.—Cross-profile of lunar crater, oo = outer plain ; ss = outer slope; ww = wreath; 
cc = inner cliff; tt = terraced inner slope; ii — inner plain; h = central hill. 
inner plain, is broken by uneven and discontinuous terraces, 
which have the peculiar habit of land-slip terraces as one 
sees them about the flanks of a plateau capped by a heavy 
sheet of basalt. From the center of the inner plain rises a 
hill or mountain, sometimes symmetric but usually irregu¬ 
lar and crowned by several peaks. From the outer plain to 
the base of the wreath the ascent is 1,000 or 2,000 feet, and 
the ascent thence to the top of the wreath may be as much 
Fig. 3.—View and section of margin of basaltic table, showing landslip terraces. 
more. The descent from the wreath to the inner plain is 
ordinarily from 5,000 to 10,000 feet, and the height of the 
central hill is 1,000 to 5,000 feet. With rare exceptions, the 
inner plain is several thousand feet lower than the outer 
plain. 
The central hill is not universally present, but appears in 
rather more than half the craters of medium size. With 
craters more than 100 miles in diameter its occurrence is 
