DIASTROPHISM AND THE FORMATIVE PROCESSES 499 



they do not hold hydrogen and helium, which abound in the sun in 

 appreciable quantities. The sum total of the gases they do hold 

 relative to the whole mass of these planets is very small. Even in 

 the case of the earth, distinctly the most massive of the solid bodies, 

 the sifting must have gone to very notable lengths. 



It is not impossible that the nuclei of all four bodies were so far 

 sifted down as to exclude essentially all the atmospheric gases, 

 and as a result their concentration fell ultimately into the precipi- 

 tate line of descent. On the other hand, it is quite possible that 

 the earth and Venus had atmospheres of some moment at all stages. 

 In the case of the moon, there seems no escape from the view that 

 its nucleus could not have formed in gaseous fashion, for the moon 

 does not even now hold an atmosphere. In its original hot diffuse 

 state a mass of so low an order as the nucleus of the moon could 

 only hold its material in the precipitate form. The atmosphere of 

 the adult Mars is so scant that its nucleus probably had no appre- 

 ciable atmosphere. It is doubtful whether Mars could even now, 

 in its full-grown state, hold the atmosphere it has if the planet 

 were heated to the point of volatilizing its stony substances. The 

 cases of Venus and the earth seem so nearly on the border line 

 that it is not unreasonable to take either view as the evidence now 

 stands. Further study may turn the scales one way or the other. 



So far as the shrinkage question is concerned, the matter 

 narrows down to the possibility that the nuclei of the earth and 

 Venus passed from their original gaseous states into planetary cores 

 along the normal line of gaseous descent. If the main mass of the 

 nucleus of the earth passed from the solar gaseous state into a cen- 

 tral liquid magma and thence by chemico-crystalline action into a 

 solid core, the process would have given special facilities for 

 the adjustment of the matter in the interest of density. To that 

 extent it would have forestalled later shrinkage that might other- 

 wise have been recorded in diastrophic features. The record would 

 not cover the full reality. The large amount of shrinkage deduced 

 by our comparison of the moon, Mars, Venus, and the earth' 

 would not be recorded even in the basal features of the earth's 

 configuration. These studies, however, imply that the unrecorded 



^Jour. GeoL, Vol. XXVIII (1920), pp. 1-17. 



