THE PAST HISTORY OF OUR MOON. 89 



It was at this stage, in all probability, that processes such 

 as those illustrated by Nasmyth's globe experiments took 

 place, and that from time to time the crust yielded at parti- 

 cular points, which became the centres of systems of radiating 

 fissures. Before proceeding, however, to consider the results 

 of such processes, let it be noted that we have seen reason 

 to believe that among the very earliest lunar formations 

 would be rifts breaking the ancient surface of the lunar crust. 

 I distinguish in this way the ancient surface from portions 

 of surface whereof I shall presently have to speak as formed 

 at a later time. 



Now let us conceive the somewhat thickened crust con- 

 tracting upon the partially fluid nucleus. If the crust were 

 tolerably uniform in strength and thickness we should expect 

 to find it yielding (when forced to yield) at many points, 

 distributed somewhat uniformly over its extent. But this 

 would not be the case if as we might for many reasons 

 expect the crust were wanting in uniformity. There would 

 be regions where the crust would be more plastic, and so 

 readier to yield to the tangential tensions. Towards such 

 portions of the crust the liquid matter within would tend, 

 because there alone would room exist for it. The down- 

 drawing, or rather in-drawing, crust elsewhere would force 

 away the liquid matter beneath, towards such regions of less 

 resistance, which would thus remain at (and be partly forced 

 to) a higher level. At length, however, the increasing 

 tensions thus resulting would have their natural effect ; the 

 crust would break open at the middle of the raised region, 

 and in radiating rifts, and the molten matter would find vent 

 through the rifts as well as at the central opening. The 

 matter so extruded, being liquid, would spread, so that 

 though the radiating nature of the rifts would still be in- 

 dicated by the position of the extruded matter there would 

 be no abrupt changes of level. It is clear, also, that so soon 

 as the outlet had been formed the long and slowly sloping 

 sides of the region of elevation would gradually sink, press- 

 ing the liquid matter below towards the centre of outlet, 



