GENERAL RELATIONS. 79 



can control, 1 the gas expelled from the molten sphere in excess of the mass 

 of the present atmosphere would escape and be lost to the planet. Geo- 

 logical evidences early Cambrian glaciation, Paleozoic periods of aridity, 

 and the general testimony of life all point toward the conclusion that 

 early terrestrial atmospheric conditions were not radically different from 

 those of to-day. If the hypothesis of a heavy atmosphere be not permissi- 

 ble, it becomes very difficult to explain the presence of original gases and 

 gas-producing compounds in plutonic rocks on the basis of the Laplacian 

 or other hypotheses that postulate original fluidity. 



RELATIVE TO THE PLANETESIMAL HYPOTHESIS. 



After the gaseous matter of the ancestral sun was shot out from the 

 solar surface to form the two arms of the spiral nebula, as postulated by 

 the planetesimal hypothesis, the rock-producing portion is supposed either 

 to have aggregated into planetesimal bodies, or to have been gathered, 

 molecule by molecule, into the nucleus of the earth. The planetesimal 

 bodies gathered in gas molecules of the atmospheric class both by chemical 

 union and by surface adhesion or occlusion. As the earth grew by sweep- 

 ing in the planetesimals, whatever gases they contained became entrapped 

 in the body of the growing planet and well distributed throughout its 

 mass. At first, the gravity of the earth may possibly have been able to 

 hold only the gases brought in by planetesimal aggregates of rock material 

 and those that became impounded in it by impact, but at a later stage, 

 when increased mass enabled it to hold gaseous molecules, gases may have 

 been added to the atmosphere directly from the nebula, and these, by 

 chemical reactions, may have become united with the surface rocks. As 

 soon as vulcanism commenced, a system of exchange was set up. While 

 gases were being fed to the atmosphere by volcanic action, water, carbon 

 dioxide, oxygen, and nitrogen were being buried with the surface rock 

 material, partly by chemical union and partly by mechanical entrapment, 

 as the growth by infalling matter continued. It is thus quite easy to 

 understand how the earth came to be affected by these gases throughout 

 its mass, and how they came to exist there in all available forms of retention. 



While the carbon monoxide and methane derived from rocks by heat- 

 ing in vacuo are doubtless chiefly produced from the carbon dioxide and 

 water present in the rock material, there seems good reason to suppose 

 that similar reactions took place within the earth, as the surface material 

 became buried and heated, and hence that carbon monoxide and methane 

 exist, as such, in the earth's body, and are to be reckoned among the natural 

 gases of the rocks. 



RELATIVE TO ATMOSPHERIC SUPPLY. 



The fact that many of the igneous rocks are able to yield hydrogen 

 from reactions between water and ferrous compounds, at high tempera- 

 tures, indicates that the material of the earth's crust is in a condition of 

 partial oxidation only. Near the center of the earth there is probably 

 very little oxygen, and even up to the surface, barring the weathered 



1 R. H. McKee, Science, vol. 23 (1906), pp. 271-274. 



