HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 95 



cules as were driven into the pores and other interstices of the frag- 

 mental surface arising from the infall of the planetesimals. 



The extent to which gases may be held condensed in small solid 

 bodies is shown by meteorites and igneous rocks to be large. Meteor- 

 ites carry, on the average, several times their volumes of condensed 

 gas; so do many, probably most, igneous rocks of the earth (Vol. I, 

 pp. 619-620). 1 The testimony of the meteorites is peculiarly significant 

 here, for they have traversed unknown depths of space in a practical 

 vacuum, in addition to the vicissitudes of their origin and the heating 

 of their fall. Atmospheric material is carried into the earth's body by 

 them to-day in quantities that are large, relative to their masses. Their 

 testimony becomes the more significant if we accept that view of their 

 origin which makes them but the fragments of small atmosphereless 

 bodies, built up precisely as the early earth was under this hypothesis. 

 This view makes them specific samples of the products of the assigned 

 process. 



The atmospheric material thus condensed within the growing earth 

 could become a part of the atmospheric envelope only by extrusion. 

 The assigned modes of extrusion will be considered presently; mean- 

 while it may be assumed that these internal gases were given forth 

 progressively, and fed the atmosphere. 



The nature of the initial atmosphere. — The contribution made 

 by the external source of atmospheric material might include any 

 constituent of the ancestral sun that could remain free in the nebula, 

 and be picked up and held by the earth. Some portion of the con- 

 stituents of the present atmosphere may therefore be assigned to 

 this source. In what ratio these constituents were contributed to 

 the nebula probably depended on their proportions in the ancestral 

 sun, or rather their proportions in that part of the ancestral sun that 

 was dispersed to form the parent nebula. Concerning this little can 

 safely be said. Hydrogen is apparently very abundant in the outer 

 part of the sun, but it is doubtful whether the earth can, even now, 

 hold hydrogen in a free state permanently in any large amount. Of 

 the proportions of the common atmospheric constituents in the sun, 

 in a free state, little is known. 



1 An ampler series of analyses of gases occluded in rocks, now being made by 

 R. T. Chamberlin, confirms previous results and seems to foreshadow the conclusion 

 that these gases are retained with much tenacity, but are given up on the complete 

 weathering of the minerals holding them. 



