3 io EARLY STAGES OF EARTH'S HISTORY 



light and too rapid evaporation, seem irreconcilable with a vast 

 cloudy atmosphere overcharged with carbon dioxide and water- 

 vapor. The hypothesis of an enormous original atmosphere, suffer- 

 ing gradual depletion, finds, therefore, scant support in a critical 

 study of either the biological or the physical history of the earth. 



Modifications of the Laplacian hypothesis (known commonly 

 as the Nebular hypothesis) have been suggested, 1 with a view to 

 obviating the objections to the current form of the hypothesis as 

 applied to the earth. But the suggested changes do not seem very 

 satisfactory, and there is reason for thinking that all hypotheses 

 of the earth's origin involving a molten condition of the globe, will 

 soon be abandoned by geologists. 



II. 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: 



1. Nuclear stage. A knot of the nebula was the nucleus of 

 earth growth. The knot may have been gaseous, or planetesimal, 

 or both. It caught planetesimals from the nebular haze as it 

 crossed their paths, and thus grew in mass while it was being con- 

 densed into the beginning of the earth-body. This stage lasted 

 until the knot was condensed into a solid mass. This mass then 

 served as the nucleus for further growth by captured plane- 

 tesimals. 



2. Initial atmospheric stage. There may possibly have been an 

 early stage when the mass of the earth was too small to hold perma- 

 nently the lighter free molecules such as form our present atmos- 

 phere. In this case the nucleus must have been made up mainly of 

 heavy molecules such as form the stony and metallic parts of the 

 earth; but such a stage is not probable. If the mass of the nucleus 

 were one-tenth of that of the present earth, it would hold some 

 atmosphere, but it would be thin and composed mainly of the 

 heavier gases. This early thin atmosphere grew as the earth grew, 

 by capturing molecules from the nebulous mass. The stony and 

 metallic planetesimals also contained atmospheric material in 

 combination or occluded, 2 and some of this, set free when the 

 planetesimals were heated by plunging into the air or when they 



1 Vol. II of the author's three-volume Geology. 



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



