THE PAST HISTORY OF OUR MOON; 91 



outflowing as the crust continued to contract would form a 

 raised wall, Until the time came when the liquid nucleus 

 began to contract more rapidly than the crust, the large 

 crateriform orifice would be full to the brim (or nearly so), 

 at all times, with occasional overflows : and as a writer who 

 has recently adopted this theory has remarked, 'We should 

 ultimately have a large central lake of lava surrounded by a 

 range of hills, terraced on the outside, the lake filling up 

 the space they enclosed.' 



The crust might burst in the manner here considered, at 

 several places at the same or nearly the same time, the 

 range of the radiating fissures, depending on the extent of 

 the underlying lakes of molten matter thus finding their 

 outlet ; or there might be a series of outbursts at widely 

 separated intervals of time and at different regions, gradually 

 diminishing in extent as the crust gradually thickened and 

 the molten matter beneath gradually became reduced in 

 relative amount. Probably the latter view should be ac- 

 cepted, since, if we consider the three .systems of radiations 

 from Copernicus, Aristarchus, and Kepler, which were 

 manifestly not formed contemporaneously, but in the order 

 in which their central craters have just been named, we see 

 that their dimensions diminished as their date of formation 

 was later. According to this view we should regard the 

 radiating system from Tycho as the oldest of all these 

 formations. 



At this very early stage of the moon's history, then, we 

 regard the moon as a somewhat deformed spheroid, the 

 regions whence the radiations extended being the highest 

 parts, and the regions farthest removed from the ray centres 

 being the lowest. 1 To these lower regions whatever was 



1 Where several ray centres are near together, a region directly 

 between two ray centres would be at a level intermediate between 

 that of the ray centres and that of a region centrally placed within a 

 triangle or quadrangle of ray centres ; but the latter region might be at 

 a higher level than another very far removed from the part where the 

 ray centres were near together. For instance, the space in the middle 



