FUNDAMENTAL PROBLEMS OF GEOLOGY. 24 1 



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.,* there would seem to be no lack of heat, in the later 

 stages at least. The essence of the problem lies in its redistribution 

 and in its selective action. 



The material of the interior was originally, b}^ hypothesis, an inti- 

 mate 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 the kind 

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

 shrinks little, 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 b}^ 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 were 

 reached. Local spots of fusion must thus arise. To this fusion the 

 entrapped and occluded gases may be presumed to have contributed 

 and to have joined themselves to the fused masses, and to have aided 

 in giving them fluidity. 



As the rise of temperature continued, more and more of the mixed 

 material reached the fusing-point, while other material so nearly ap- 

 proached it as to become plastic and permit readjustive movements. 

 In this way fused points are supposed to have been permitted to join 

 one another and to move in the direction of least resistance. The 

 static pressure from the earth body itself was always greatest below 

 and least above, but was nearly constant for any given short period. 

 The stresses arising from the differential tide-producing attractions 

 of the sun and moon ^vere also greatest below and least above, but 

 were periodic, stress and relief following one another in semi-daily 

 succession, giving a kind of kneading process. These interior stress 

 differences are thought to have pressed outward the fused vesicles, 

 causing them to unite and form threads or stringlets, insinuating 

 themselves through the more refractory portions that remained solid, 

 and at length developing into tongues of some volume; As these 

 liquid threads or tongues rose to higher horizons of lower pressures, 



^ See the investigations of Dr. Lunn. 



