DIASTROPHISM AND THE FORMATIVE PROCESSES 683 



Let us now reverse the mode of inquiry by trying to approximate 

 a rate of infall that would cause the melting of the earth surface, 

 and then compare results with those reached in the preceding ways. 



If the mass of the earth-core equaled one-third that of the 

 present earth, an atmosphere of sufficient depth to protect its sur- 

 face from the direct impact of planetesimals of the specified mass 

 would have surrounded it. The melting of the earth must then 

 have hung upon the competency of the infall to so heat the upper 

 atmosphere as to melt the earth surface some miles below. About 

 half the heat acquired by the thin upper air would have been quite 

 promptly radiated outward and the melting left to the other half. 

 The effect of the air on meteorites plunging into it is suggestive 

 in this connection. As soon as a film of meteorite-substance 

 becomes viscous enough to yield to the high pressure of the air 

 condensed on the meteorite's front by its high speed, the film is 

 driven backward and dissipated along the meteor's path forming 

 the "streak" of the "shooting star." Only a very small part is 

 melted at any one instant, or left in any one spot. Even this 

 minute part only reaches the first stages of the molten state and 

 hence is very quickly cooled again to the solid state. To apply 

 this to planetesimals, it is to be noted that the mean velocity of 

 meteorities is probably four or five times that of normal planet- 

 esimals, and their moving energies sixteen to twenty-five times as 

 great in proportion to mass. The working picture, then, in the 

 case in hand, is that of a little mass, one-fiftieth of a pound, mak- 

 ing a similar but feebler streak of quickly heated, quickly cooled 

 matter, down the center of a column of air one square foot in 

 cross-section. This must take place in such close succession as to 

 melt one square foot of the earth surface at the bottom of the 

 atmosphere in spite of outward radiation. To really complete the 

 picture, it is necessary to add that the lower atmosphere would 

 soon be filled with the dust of the dissipated planetesimals and the 

 melting of the surface would have to be effected through this 

 screen. It seems clear that to effect general melting the upper 

 atmosphere must be heated throughout to the melting-point of 

 average rock-substance, and kept at that temperature in spite of 



