47 8 Proceedings of Royal Society of Edinburgh. [sess. 
ploughed out by the grazing impact of a moonlet. Its floor is 
level with Mare Imbrium, and its sides are nearly vertical ; hence 
it may have been scoured through by white-hot lava from the 
Mare Imbrium when that mare was violently surging on its for- 
mation. There are numerous features of the nature of the Valley 
of the Alps on the moon’s surface, notably in the region of craters 
Albategnius and Ptolemseus, and also in the region south of Mare 
Serenitatis. These are arranged in series of parallel lines, and 
may be due to the grazing impact of swarms. 
A large portion of the moon’s surface is covered with the maria, 
some of which have a roughly circular outline, such as Mare 
Imbrium, Mare Serenitatis, and Mare Crisium, which seems to 
indicate that each is the result of some single great catastrophe. 
These may have been formed by the impact of a nebula or swarm 
of bodies ; and the mountain ranges bordering them, such as the 
Alps and Apennines bordering Mare Imbrium, may have been the 
result of the same catastrophe which formed the sea they are 
associated with. These mountain ranges have all the appearance 
of masses of matter thrown down in a sidelong heap and splashed 
over with liquid rock. There is much appearance on the surface 
of the maria of their having been in commotion, and indeed they 
must have been in violent commotion when they were formed. 
Many long ridges on their surfaces show that they have not quite 
come to a level surface till they were too viscous to do so. These 
ridges seem to indicate a creeping together of the lava from 
opposite sides when it was nearly solid. The surfaces of the maria 
are generally darker than the rest of the moon’s surface, owing, no 
doubt, to their comparative smoothness rather than to any differ- 
ence in the kind of rock ; obviously, a polished surface would look 
black at full moon, if not at the centre of its disc. 
A very interesting feature, that may be noticed more or less on 
all parts of the moon’s surface, is the immense number of old 
craters and mountain ranges that have been overwhelmed by the 
lava of the maria, or battered down by more recent formations ; 
which shows that the formation of those craters and maria is no 
casual occurrence, depending on the chance meeting of meteors 
from outer space, but the natural process by which the moon’s 
mass has been built up. 
