276 
GILBERT. 
of a deluge of material—solid, pasty, and liquid. Toward 
the southwest the deluge reached nearly to the crater The- 
ophilus, a distance of 900 or 1,000 miles, and southward it 
extended nearly to the latitude of Thebit. Northward and 
northeastward it probably extended to the limb. Westward 
it passed beyond Posidonius, and toward the east and south¬ 
east its traces are lost in the Oceanus Procellarum. Its more 
liquid portion gathered on the lowlands, giving rise to sev¬ 
eral maria and minor plains. The fact has been recognized 
by various students, notably by Green * and Meydenbauer,f 
that many of the lunar plains are due to floods of molten 
material overspreading the low lying tracts and burying 
the preexistent irregularities of surface. At various points 
in such plains, and especially at their margins, crescentic 
hills project above them, recognized as portions of crater 
rims; and elsewhere the plains are divided by systems of 
cracks whose arrangement betrays the distribution of under¬ 
lying ridges. The plains most closely associated with the 
sculpture system and the supposed viscous deposit are the 
Sinus Roris, Mare Frigoris, Lacus Mortis, Lacus Somniorum, 
Palus Nebularum, Mare Tranquilitatis, Mare Vaporum, 
Sinus Medii, Sinus Estuum, and Mare Nubium. The Oceanus 
Procellarum may have been created at the same time or 
may have been merely modified by this flood. The Mare 
Serenitatis, whose sharp outlines and circular form mark it 
as an old crater, doubtless received a new surface. 
As to the precise nature of this catastrophe I am in doubt. 
Its focus lies within the great crater rim of the Caucasus, 
Apennine, and Carpathian ranges, but is not concentric with 
that rim, and it is not surrounded by a rim of its own. 
The lofty plateau lying north of the Mare Imbrium, although 
presenting a steep face toward the mare and a long slope in 
the opposite direction, has not the simple contour of a crater 
wall, but is variously notched. By considering the extent 
and probable thickness of the various deposits from the 
*E. N. Green: Jour. Brit. Ast. Ass., April, 1891, p. 379. 
f A. Meydenbauer: Sirius, February, 1882. 
