64 ILLINOIS ACADEMY OF SCIENCE 
universal ocean overhung by a still deep moist atmosphere ; and 
then (4), a series of slow internal and external changes. Hav- 
ing grown thus gradually out of the fluid state, the concentric 
arrangement of material normal to fluidity should have domin- 
ated the whole evolution and have had marked expression in 
the final product presented for inspection and verification 
today. 
The series of planetesimal pictures departs widely from 
these: A nebular knot formed a nucleus at the start, partly 
gaseous, partly orbital, amounting, it may be, to a third or a 
half of the final mass, An early concentration of the knot 
into an earth-nucleus was followed by a very slow growth from 
the scattered planetesimals afterwards. The long series of 
infalls of planetesimals generated much heat, but chiefly in the 
upper zones of the growing atmosphere, whence it was easily 
and promptly radiated away. The magnetic and inelastic 
material was brought in faster than the non-magnetic and 
non-elastic, because magnetic attraction supplemented gravita- 
tive attraction, and because the orbital motions of the inelastic 
planetesimals were faster reduced by mutual collisions than 
those of the elastic planetesimals. And so the metals and the 
basic rock-material gathered more largely toward the center, 
while the more elastic material gathered later and more largely 
into the outer parts. The accessions were very heterogeneous 
notwithstanding. The planetesimals plunging into the atmos- 
phere became ignited by the stroke and were largely dissipated 
to dust which floated at the will of the winds until gravity or 
precipitation brought it to the ground or the sea. This tlota- 
tion had a more or less sifting effect, separating in some 
little degree the heavier from the lighter material, thus building 
into the very body of the earth a differentiation of specific 
gravity. This differentiation was abetted by the growing 
hydrosphere, and both atmosphere and hydrosphere were lo- 
calized in their activities by the increasing deformations of the 
earth body. These united processes led thus on to continental 
embossments of lighter materials, and to abysmal sags of 
heavier materials. Radioactive matter, lodging at first pro- 
miscuously in the heterogeneous mass of the growing body, 
formed a multitude of self-heating centers. These, co-operat- 
ing with other sources of heat arising variously in the self- 
compressing body, started local liquefaction in such material as 
