104 GEOLOGY. 



weight overbalanced the differential pressure, such as metallic iron and 

 possibly the heaviest silicates, may be regarded as forming exceptions. 



Arrested ascensions and attendant heating. — Theory does not 

 require that these threads should all succeed in reaching the surface; 

 indeed, it does not require that any should, in the initial stages, before 

 compression had developed a great excess of heat in the central parts. 

 The molten threads should simply rise until their excess of heat, their 

 working capital, was exhausted, when they would return to the solid 

 state and constitute tongue-like intrusions. In doing this, they would 

 contribute heat to the tracts which they invaded. This, in addition to 

 conduction, was a mode of conveying the intenser heat of the com- 

 pressed central regions to the higher horizons, where the original tem- 

 perature and the fusion-points were both lower. The failure of the 

 earlier threads to reach the surface would thus be a means prepara- 

 tory to the greater success of later ones. The conditions for pene- 

 tration would probably be favorable up to the horizon where the tem- 

 perature ceased to be higher than the surface melting-point. Below 

 this the retention of the solid state was wholly due to pressure, the 

 temperatures being above the surface melting-point. When the 

 threads reached the higher zone, in which the temperature was appre- 

 ciably below the surface fusing-point, the conditions were clearly 

 adverse, and further ascent was dependent on a sufficient excess of 

 heat, brought from below, to maintain the liquid state while this 

 adverse tract was being traversed. It was probably also dependent 

 on a fluxing power adequate to enable it to fuse its way through the 

 solid zone of continuous rock that lies below the fracture zone. When 

 it reached the latter, hydrostatic pressure and the inherent expansive 

 force of its gaseous content would probably control its further course, 

 in the main. 



Now, having in mind that, at the early stage under consideration, 

 the earth was growing, that its internal self-compression was increasing 

 apace with its growth, that the heat was rising with the compression, 

 that the temperature was highest at the center and graded towards 

 the surface, that the heat was therefore always flowing toward the sur- 

 face, and that it was also carried outward by the liquid threads, the 

 succeeding steps may be followed easily. 



Invasion of the fragmental zone and surface explosions. — The outer 

 part of the young earth was made up of the recently fallen planetesi- 



