THE PAST HISTORY OF OUR MOON. 87 



must assume that the matter forming the solid nucleus at 

 that early stage was relatively less in amount, or else that 

 we may attribute part of the difference to the comparatively 

 small force with which lunar gravity operated during various 

 stages of contraction and solidification. 



In the case of the moon, as in that of the earth, before 

 the last portions became solidified, there would exist a con- 

 dition of imperfect liquidity, as conceived by Hopkins, 

 4 preventing the sinking of the cooled and heavier particles, 

 and giving rise to a superficial crust, from which solidification 

 would proceed downwards. There would thus be enclosed 

 between the inner and outer solid parts a portion of uncon- 

 gealed matter/ which may be supposed to have retained its 

 liquid condition to a late period, and to have been the prin- 

 cipal seat of volcanic action, whether existing in isolated 

 reservoirs or subterranean lakes, or whether, as suggested 

 by Scrope, forming a continuous sheet surrounding the solid 

 nucleus. 



Thus far we have had to deal with relations more or less 

 involved in doubt. We have few means of forming a satis- 

 factory opinion as to the order of the various changes to 

 which, in the first stages of her existence as a planet, our 

 moon was subject. Nor can we clearly define the nature of 

 those changes. In these matters, as with the corresponding 

 processes in our earth's case, there is much room for variety 

 of opinion. 



But few can doubt that, by whatever processes such con- 

 dition may have been attained, the moon, when her surface 

 began to form itself into its present appearance, consisted 

 of a globe partially molten surrounded by a crust at least 

 partially solidified. Some portions of the actual surface 

 may have remained liquid or viscous later than others but 

 at length the time must have arrived when the radiating 

 surface was almost wholly solid. It is from this stage that 

 we have to trace the changes which have led to the present 

 condition of the moon's surface. 



It can scarcely be questioned that those seismologists are 



