246 H. L. FAIROHILD GEOLOGY UNDER PLANETESIMAL HYPOTHESIS 



ocean were originally a part of the planetesimals and helped to form the 

 earth's mass. Whether the elements were superficial condensations on 

 the solid planetesimals, like the occluded gases of meteorites, or formed 

 part of their essential substance may not be important to the present 

 discussion. The really important principle is that these substances 

 were world-stuff, and that they were carried into the earth's mass by the 

 accreting process of earth-making. A large portion of the nitrogen, 

 oxygen, hydrogen, carbon, helium, argon, and other substances has 

 been subsequently forced from the earth's interior to the surface by 

 gravitational condensation and the resulting heat. This out-squeezing 

 of the volatile substances has been a continuous process ever since an 

 early stage of earth accretion. But the atmosphere did not exist until 

 the earth had reached a size somewhat larger than the moon, for it 

 appears, according to the laws of kinetics, and judging from the present 

 naked condition of the moon, that up to that size the gravitational attrac- 

 tion of the earth was not competent to hold the gases on its surface. 

 Even today the molecular velocities of hydrogen and helium seem to be 

 sufficient to carry the molecules of those gases beyond the earth's effi- 

 cient attraction. The atmosphere and the ocean of today are only such 

 portions of the gaseous emanations from the earth's interior as the earth 

 has been able to hold within its grasp, minus the considerable part 

 which has been restored to the superficial lithosphere by carbonation 

 and oxidation processes. 



The atmosphere has had a slow growth, from a probable film of car- 

 bon dioxide to its present volume, and the growth is still in progress 

 through volcanic and other exudations and by release of gases in decay 

 of the crystalline rocks. Throughout geologic time, as recorded in the 

 stratified rocks, loss and supply of the carbon dioxide seem to have been 

 fairly well balanced, as Professor Chamberlin and other writers have 

 shown how counterbalancing activities have probably served to check 

 great excess of this chemically active substance on the one hand or its 

 great depletion on the other and how the ocean has acted as a reservoir 

 and equalizer. In the case of nitrogen, there would seem to have been 

 little loss on account of its chemical inertness. The relative proportion 

 of oxygen is an interesting question. The suggestion of several writers, 

 including Lord Kelvin, that the presence of free oxygen in the air is due 

 to the action of sunlight on plants, is opposed, by the requirement in 

 oxygen of the earliest animal life. The relative intensity of oxidation 

 processes in early as compared with later geologic periods is a new prob- 

 lem under the new hypothesis. Such knowledge as we have relating to 

 geologic climates seems to indicate that the atmospheric conditions of 

 post-Archean time were not radically unlike those of today. But the 

 primitive atmosphere must have been very different, as, theoretically, it 



