102 GEOLOGY. 



The hypothetical mode of vulcanism. — With the detailed con- 

 ceptions now developed, the method of volcanic action, deduced from 

 the accretion hypothesis, may be readily apprehended, and the vital 

 part which it began to play in earth-history may be realized. The 

 chief portion of internal heat being assigned to compression, the tem- 

 perature must have been highest at the center and must have declined 

 toward the surface. By referring to pages 563-569, Vol. I, the dis- 

 tribution of pressures, densities, and temperatures for the adult earth 

 will be found. These elements must have had the same order of dis- 

 tribution, in a general way, for any of the immature stages of the 

 earth, but there should have been some differences in proportionate 

 value because sources (1) and (2) named above would have become 

 negligible elements as the earth grew to its maturity. 



Melting due to outward flow of heat.— Pressure itself is probably 

 incompetent to melt rock-substances that shrink in solidifying, but 

 the high temperatures generated by pressure in the deep interior were 

 constantly moving outward into horizons of lower pressures where 

 the melting-points were lower. As the computed temperature at the 

 center of the adult earth is about 20,000° C., 1 there would seem to be 

 no lack of heat, in the later stages at least. The essence of the prob- 

 lem lies in its redistribution and in its selective action. Its initial 

 distribution and its outward flow have been set forth in the uages 

 referred to in Vol. I. 



Selective fusion. — The material of the interior was originally, by 

 hypothesis, an intimate mixture of planetesimals of various kinds, 

 with such gaseous material as they carried in, or entrapped, in the 

 process of growth. This material, therefore, presumably ranged from 

 the most fusible to the most infusible of rock-material that could take 

 the form of aggregated planetesimals. As some of it was probably 

 of the kind that shrinks much in solidifying, and some of the kind that 

 shrinks little in solidifying, and some, possibly, of the kind that does 

 not shrink at all in solidifying, it is probable that some of it was brought 

 near or even to the melting-point by pressure, while other parts, 

 intimately intermixed with these, were far from their melting-points. 

 At any rate, the outward flow of heat, in such a mixture, must bring 

 some parts to fusibility much before the melting-points of other parts 



1 A more elaborate discussion of internal temperatures due to compression, by 

 Mr. Lunn, will soon be published under the auspices of the Carnegie Institution. 



