GEOLOGY. 413 



type. It will be easy to modify the deductions to fit special views, 

 if desired. 



A working difference is to be noted at the outset: On the one hand, 

 thermal or vibratory forms of energy dominated the compressional 

 gaseo-molten descent; on the other, the properties of solidity — herein 

 interpreted as due to revolutional motion — dominated the process 

 during planetesimal growth; the two modes are therefore more nearly 

 antithetical than coincident. 



a. Making the usual assumptions in both cases, viz, that the earth 

 was formed in its present position and relations in the solar system and 

 that the established laws of dynamics were in full control, the original 

 dispersion of the earth-forming planetesimals, conservatively estimated, 

 was 9 X 10"^ cubic miles, while that of the earth-forming gas, liberally 

 estimated, at the time when its own gravity first came into control 

 of it, was about 3.5 X 10^^ The ratio of the former to the latter 

 was thus roundly 250,000 : 1. 



b. For present purposes it is easiest to compare energies in terms 

 of velocity simply, for the mass we are dealing with is the same in all 

 cases. The earth's rotation involves a mean velocity of only a small 

 fraction of a mile per second. It may be left out of account in the 

 comparison, though it amounts to nearly 16 X 10'^ foot-pounds. The 

 mean revolutional velocity of the earth may be taken roundly as 18 

 miles per second, while the potential energy relative to the sun is 

 about 356 miles per second. Motions and positions relative to star- 

 clusters and the stellar galaxy, as also whatever may lie beyond, are 

 here neglected because there is no adequate basis for estimating them. 

 The additional energy involved in these is probably very large. 



The molecular velocities of the earth in a gaseous state could not 

 have been as high as 7 miles per second without dispersing the mass, 

 since 6.95 miles per second is the parabolic velocity at the present earth 

 surface. If, therefore, we take 7 miles per second as the mean velocity 

 of the vibratory action of the molecules of the earth, it will be ex- 

 cessive. 



The revolutional energies are therefore represented by 18^H-356^, 

 neglecting a large unknown value, while 7^ is an over-generous repre- 

 sentative of the vibratory energies. The ratio of the former to the 

 latter is about 2600 : 1. 



c. It required perhaps two or three billion years for the gathering 

 in of the planetesimals, for they were controlled by concurrent revolu- 

 tional dynamics. The gaseous nebula was of course controlled by 

 gaseous dynamics and would have collapsed as fast as the radiation 

 of the heat pennitted, occupying not more than a few million years at 

 most. 



There was an enormous loss of energy during the concentration 

 process in both cases. The availability of the remaining energies for 



