810 



DYNAMICAL GEOLOGY. 



7,790 pounds and 11,880 pounds to raise the fusing point one degree 

 than it does below 7,790 pounds, and hence they sustain, as he has 

 urged, the idea of such a limit. 



(2.) Again, it has been argued that since hardened rock is denser 

 than fused, and, therefore, any portion that became solid at surface 

 with the progress of cooling would sink and be refused; that this 

 process would continue until the whole interior, to the very centre, was 

 too stiffly viscid or pasty to allow of further sinking ; and only then 

 would a permanent crust begin to form. 



But, as Professor Hennessy observes, the materials in fusion would 

 arrange themselves in strata according to their different degrees of den- 

 sity, and hence, in the progressing solidification, the hardened rock would 

 sink only to a limited distance beneath the surface, and the circulation 

 would be comparatively superficial ; that thus a crust would finally be 

 formed with a pasty layer or region beneath it, and below this there 

 might be complete fusion to the centre. Mallet sustains the same con- 

 clusion on the ground that slags form over a fused mass of the slag- 

 material at a temperature very near that of fusion. Muirhead, in his 

 experiments with reference to the point here considered, found that 

 when a bar of cold iron was dropped endwise on fused iron, it bounded 

 back to the surface, and melted there ; and that when masses of trap 

 (whinstone), and also of cold furnace slag, of five or six pounds weight, 

 were placed on fused slag, they at first sank, but soon came to the 

 surface and floated about until they were melted. Further, owing to 

 escaping vapors attending the igneous fusion, the first cooled material 

 would be light from air-vesicles, like scoriaceous lava. 



It is also an important consideration that the study of meteorites has led some astron- 

 omers and writers on the subject to the opinion, in view of the iron in these bodies, and 

 of the fact that their place in the solar system is to a large extent near that of the earth, 

 that the earth's interior consists, for the greater part, of iron. This view is favored, also, 

 by the high percentage (10 to 14) of iron oxyd in most igneous rocks; the existence of 

 much native iron in doleryte at a locality in Greenland; and the occurrence of the great- 

 est of iron ore beds of the world in the oldest rocks, the Archaean. Platinum, gold, sil- 

 ver, and copper are heavier metals; but it is remarkable that they are not brought up 

 among the constituents of eruptive rocks, as iron is, but are obtained from the supercrust 

 and its veins: as if these metals, in consequence of being in vaporizable combinations, or 

 those of comparatively little specific gravity, were near the surface of the fused globe, 

 while below these were the iron and what, under the conditions, could form alloys with 

 it. If the earth is two thirds iron, or iron to within 500 miles of the surface (without 

 much increase in the density of the iron downward), and the rest were made of dolerytic 

 material, it would have about its present specific gravity, 5 - 5-6. With such a nucleus, 

 the stony material, made as cooling went on at surface, could not sink down into the 

 iron ; and not to it, because of the density of an overlying region. Whether the iron 

 would be in a solid state or a liquid, science may yet determine. 



3. The demands of Geological Facts. — Geological facts relate to 

 the crust and supercrust, and do not touch the question as to the earth's 



