FUNDAMENTAL PROBLEMS OF GEOLOGY. 25 1 



In the early stages the danger seems to be all on the side of too great 

 heat. Even if the sun's heat were much less than now, the heat of 

 planetesimal infall would probably make up the deficiency and more. 

 The infall would continue to be a source of home supply so long as 

 the accretion continued, declining as the supply of planetesimals 

 diminished. This diminution of the supply cleared the space between 

 the earth and the sun, and gradually brought the sun into full function. 

 There would, therefore, be a gradual passage from the partial de- 

 pendence on the tiome supply of heat and light to complete depend- 

 ence on the solar supply. There is little ground for apprehension 

 that the infalling planetesimals would be seriously dangerous to the 

 early forms of life, for in the first place the atmosphere must have 

 been then, as now, an effective cushion, checking the speed of the 

 planetesimals and partially dissipating them, and, in the second place, 

 the early organisms were probably all aquatic and were further pro- 

 tected by their water covering. 



The introduction of organic activity is presumed to have brought 

 into play the well-known attendant chemical processes. The changes 

 in the composition of the atmosphere are especially important. It 

 has been indicated that the primitive atmosphere probably contained 

 a preponderance of carbon dioxide, and a little later carried all the 

 water-vapor it could hold under the prevailing temperatures, while 

 the amount of nitrogen was not improbably low, and that of oxygen 

 uncertain. If only there were oxygen enough to serv^e the functions 

 of plant life at the outset, the existing large content of oxygen could 

 probably all arise from subsequent plant action. It is merely neces- 

 sar>', therefore, to assume (i) that the carbon dioxide was not too 

 abundant to prohibit the development of the early plants ; (2) that 

 the oxygen was sufficient for their vital processes; and (3) that the 

 nitrogen was much less abundant than now, to give a good working 

 basis for the evolution of the present very different atmosphere. 

 Assuming that green (photogenetic) plants were first introduced, 

 and that until some time later there were no animals or predaceous 

 plants which decomposed the carbon compounds produced by the 

 green plants,, the first effect of the plant life on the atmosphere 

 would be to reduce its carbon dioxide and increase its free oxygen. 

 If there were no check or offset to this process, a relatively short 

 time would suffice for the conversion of an atmosphere of dominant 

 carbon dioxide to one of dominant oxygen. If the present vegeta- 

 tion can remove the present content of carbon dioxide in 100 years, 

 as estimated, an amount of carbon dioxide as great as the whole 



