FUNDAMENTAL PROBLEMS OF GEOLOGY. 243 



being traversed. It was probably also dependent on a fluxing power 

 adequate to enable it to fuse its way through the solid zone of con- 

 tinuous 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 in- 

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

 compression, that the temperature was highest at the center and 

 graded toward the surface, and that it was also carried outward bj^ 

 the liquid threads, the succeeding steps may be followed easily. 



The outer part of the young earth was made up of the recently 

 fallen planetesimals and their fragments, and no doubt had a much- 

 broken, open texture. If there was as yet no atmosphere nor hy- 

 drosphere, as in the case of the moon, there was no effective process 

 for the wash of fine fragments into the interstices of the coarse, 

 or, what is more important, for the solution of the material at the 

 surface and the cementation of that below into a solid mass, as is the 

 present habit on the earth ; in other words, there was no effective 

 healing process to unite the broken fragments. The porous clastic 

 zone must therefore have extended downward to a depth at which 

 gravity was able to force the fragments into continuity by its crush- 

 ing effects. In a small body this zone would be deep. 



When the rising lava tongues reached this outer fragmental zone, 

 fluxing was no longer required, as they could force their way by in- 

 sinuation and by mechanical displacement. It appears almost certain 

 that in the upper part of such a fragmental zone the interstices 

 would make up a sufficient part of the volume of the aggregate mass 

 to reduce its average specific gravitj^ to a figure below that of the 

 penetrating lava, even though the latter might be made up of lighter 

 material inherently, and was also hot and liquid. The earliest 

 tongues of molten material are supposed, therefore, to have generally 

 lodged within the fragmental zone, taking various plutonic forms, as 

 dikes, sills, laccoliths, and batholiths, and to have there given off 

 their gases, which, more or less concentrated and condensed, doubt- 

 less not infrequently forced an exit to the surface b)' blowing away 

 the overlying fragmental material. The slight coherence of this 

 material, the low gravity of the young earth, and the absence or 

 scantiness of a resisting atmosphere should combine to give to the 

 pit-forming effects extraordinary magnitude, such, perhaps, as the 

 moon exhibits. 



