432 GEOLOGY 



began with the deposition of the proterozoic sedimentaries, and 

 continues to the present. 



III. STAGES OF GROWTH UNDER THE PLANETESIMAL HYPOTHESIS 



It is possible to suppose that the earth grew up by accessions 

 in some other mode than that of planetesimal evolution, but the 

 latter furnishes the basis for the following sketch of probable stages: 



(1) The nuclear stage, which started with the nebular knot and 

 proceeded by its gradual condensation as planetesimals were gath- 

 ered into it, until it became a small planet which continued to 

 grow by the capture of more planetesimals. No specific mass is 

 assigned to the nuclear knot save that its mass was sufficient to 

 control its constituents. 



(2) The initial atmospheric stage. There may have been a stage 

 after the nucleus was condensed during which the earth was too 

 small to hold the gases of an atmosphere, but if the earth then had 

 a mass one-tenth or more of its present mass, it probably had a 

 limited atmosphere like that of Mars; if it was much smaller than 

 this, it probably had little or no atmosphere. Whatever the atmos- 

 pheric state at the start, it is assumed that as the earth grew, 

 atmospheric molecules were gathered to it, and sooner or later it 

 gained the power to hold them, and accumulate an atmosphere. 

 Gaseous molecules should have come in from without, the same as 

 other planetesimals, and gases should have been given forth from 

 the rock material that went to form the growing earth, particularly 

 after volcanic action had set in. The heavier gases should have 

 been retained at an earlier stage, while the lighter ones came under 

 control later. 



Gases or gas-producing material was no doubt contained in the 

 original planetesimals, since gases are given forth from nearly all 

 rocks and meteorites to-day when heated in a vacuum, 1 and as 

 the planetesimals were aggregated and heated, a part of their gase- 

 ous material should have escaped and become a part of the atmos- 

 phere. Because the presence of these gases and gas-producing 

 materials in rocks is so general, it is thought probable that many 



1 The Gases in Rocks, R. T. Chamberlin, Carnegie Institution, 1908. 



