240 CARNEGIE INSTITUTION OP WASHINGTON. 



to its surface. The amouut of this condensational heat for the full- 

 grown earth, computed on the best data now available, seems to be 

 ample to meet all the requirements of the known geologic ages, as 

 brought out in the investigations of Dr. lyuun.* That heat arising 

 from condensation solely would reach the melting temperature of 

 rock in a body one-twentieth of the earth's mass seems more or less 

 doubtful, but in a body one-tenth of the earth's mass the required 

 conditions would probably be reached. The requisite data are too 

 imperfect for a definite decision of this point at present. If the pits 

 of the moon (JL. of the earth's mass) represent volcanic explosions, 

 and not the infall of planetoids as Gilbert suggests,! it is necessary 

 to postulate in its case conditions very favorable to the generation of 

 heat by compression, or else to assign some notable portion of the 

 requisite heat to the quasi-gaseous condensation of the nucleus, to 

 the collisions of planetesimals, and to the source next to be con- 

 sidered, all of which would necessarily contribute something to the 

 sum total of internal heat. 



Another source of heat lay in the atomic and molecular rearrange- 

 ment of the material after it became entrapped in the growing mass. 

 This was not simply chemical recombination, as usually understood, 

 but molecular readjustment under pressure as well. The planet- 

 esimals were aggregated, by hypothesis, in a vacuum of the highest 

 order, and with -very slight mutual gravity, and the mode of molec- 

 ular arrangement was that suited to this extremely low pressure. 

 Under the rising pressure of the earth's interior, new arrangements 

 of the molecules into denser forms with lower specific heats are 

 theoretically assignable, if not inevitable, with the freeing of heat as 

 a consequence. In a sense this is a mode of condensation falling 

 under the previous head, but it is not identical with mere mechanical 

 compression and is not wholly covered by computations based on that. 



With the detailed conceptions now developed, the method of vol- 

 canic action deduced from the accretion hypothesis may be readily 

 apprehended and the vital part assigned to it in earth history may be 

 realized. The chief portion of internal heat being assigned to com- 

 pression, the temperature must have been highest at the center, be- 

 cause the compression was greatest there, and must have declined 

 toward the surface. 



Pressure itself is probably incompetent to melt rock substances that 

 shrink in solidifying, but the high temperatures generated by pressure 



* See statement appended to this report. 



fBiill. Phil. Soc. Washington, Vol. XII, 1892, pp. 241-292. 



