672 Me (Cy CLELANIMETEIRICION, 
would be deformations and igneous extrusions as on the earth 
today. Perhaps the reduction of metallic oxides and the working 
of the slag toward the surface may have been an incident of the 
process. Now whenever igneous extrusions invaded the zone of 
congealed vapor conditions would be afforded favorable for the 
generation of great quantities of steam temporarily restrained by 
the overlying fragmental mass and facily subject to explosive 
discharge. The peculiar constitution of such a body invites the 
notion of exceptionally explosive eruptions, as’ do also the 
extraordinary pits of the moon. Asa matter of fact the sug- 
gestion arose from studying the pits and not from the peculiar 
constitution of the body to which the speculation had led. 
These remarkable cavities seem to be the close analogues of 
the few explosive craters which the surface of the earth pre- 
sents. 
The pre-atmospheric stage of the evolution would obviously 
cease when the growing earth acquired a size sufficient to measur- 
ably control its exhaled and ingathered gases. A certain meas- 
ure of control was incidental to all stages, for even a small 
planetoid has some power to control gases of very low initial 
velocity if it continues at low temperatures. At the size of the 
moon gases of much more than the average molecular velocity 
Of those of the earth at oc ©: could be held it them velocires 
were not exalted by interaction. This exaltation would become 
ineffective when the gases became extremely rare and the sur- 
face very cold. The molecular argument does not therefore affirm 
the total absence of an atmosphere on the moon, but rather on 
the contrary its scanty presence. An effective control would 
perhaps begin to be gained by the growing planet when the size 
of Mercury or thereabout was attained. After this the vapors 
and gases of lower molecular velocity would collect upon the 
surface and initiate the appreciable history of the external atmos- 
phere. Whensoever the accretions of this atmosphere acquired 
the power of retaining the heat of the sun to such a degree as 
to give a surface temperature above the freezing point, the inau- 
guration of the hydrosphere would take place and with its pro- 
