202 H. A. Daly — Abyssal Igneous Injection. 



would become approximately 2*84. Barus lias further shown 

 that, so far as density is concerned, temperature and pressure, 

 as these increase with depth on the normal earth gradients, 

 nearly counterbalance each other's effects in glass, though 

 the density should be somewhat increased with increase of 

 depth.* It is not a violent assumption that the same be 

 taken as true for rock-matter generally. The average density 

 of the whole crust is thus nearly equal to the average density 

 of its rocks, provided these are at surface temperature and 

 pressure. The average specific gravity of the visible crust is 

 not far from 2*75 ; but if the specific gravity of the whole crust 

 were as high as 2*80, it might still float on the compressed gab- 

 broid substratum. 



The meagreness of existing experimental data will not allow 

 that statements concerning crustal flotation can be other than 

 in the conditional mood. It is certain that the relation between 

 pressure and the volumetric diminution of a fluid cannot be 

 linear for indefinitely great pressures. Yet the foregoing crude 

 estimates clearly suggest the possibility that crustal foundering 

 could not take place if the crust were even considerably thin- 

 ner than it is to-day. When the difference in the compressi- 

 bility of solid and liquid lava at great pressure is, in the 

 future, once determined, it will first become possible really to 

 test Kelvin's view as to the original cooling of the earth. The 

 analogy of Barus's experiments raises the suspicion that the 

 fragments of a foundering primal crust could sink only a 

 comparatively few miles into the liquid interior. They would 

 soon meet therein a level where their own density is matched 

 by that of the more compressible fluid. Solidification by cool- 

 ing would therefore not progress from the earth's center out- 

 wards, but would begin in a surface shell and slowly progress 

 inwards. 



Stresses within the crust. — It seems, accordingly, best to 

 conceive of the earth as exteriorly composed of a thin solid 

 shell characterized by tangential compression, a thicker under- 

 lying solid shell characterized by tangential tensions, and a 

 liquid, gabbroid, perhaps highly viscous substratum of unknown 

 thickness. This substratum is immensely elastic as to volume 

 and is compressed by the weight of at least 25 miles of crust- 

 rock. Beneath the surface shell of tangential compression the 

 rate of secular cooling and contraction and the consequent 

 tension increase from the level of no strain downwards all the 

 way to the substratum. In his first paper Davison calculates 

 that the average rending stress in the lower shell is, after a 

 given time (if there be no relief by stretching or by cracking), 

 four times the average compressive stress in the upper shell. 

 *C. Barus, Bull. 96, U. S. G. S., p. 61, 1892. 



