316 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



Miles from surface. Density. 



1372 7-09 Zinc, Iron, Antimony 



1716 7.85 Cobalt, Steel. 



2059 8-47 Uranium, Nickel. 



2402 8-96 Copper. 



2745 9-31 



' 3088 9-51 



3432 9-59 Bismuth, Silver. 



The theory maintained by Sartorius Von Waltershausen regard- 

 ing the constitution of the earth's interior, (in opposition to 

 Bunsen's hypothesis of the two separate reservoirs of acid and 

 basic molten rock,) is indicated by the above series of calculated 

 densities. He supposes that from the interior of the earth's 

 crust to its centre a gradual increase of density takes place in the 

 fluid mass, or that this fluid mass in its present condition, as in 

 former ages, consists of a series of concentric layers of molten 

 matter, which are the denser the nearer they approach to the 

 earth's centre. Instead therefore of regarding trachytic and 

 basaltic lavas as the products of the two separate reservoirs, he 

 considers them as the products of two different concentric layers, 

 Or as originating from two different levels in the fluid mass, the 

 basaltic lava cccupyingthe lower layer or level, and the trachytic 

 floating above it ; the one, both as regards chemical composition 

 and density, graduating into the other. Von Waltershausen found 

 the mean specific gravity of seven different Icelandic trachytes to 

 be 2,524, while that of ten different basaltic lavas amounted to 

 2,91 1. With the increase of specific gravity towards the centre, Von 

 Waltershausen supposes also an increase in the basic constituents 

 of the molten rock, a change from a purely feldspathic material, 

 yielding trachytic rocks mainly composed of feldspar, to one 

 much richer in lime, magnesia and "iron-oxide, and yielding 

 dolerites consisting of feldspar, hornblende or augite, and in 

 smaller quantity magnetic iron ore. He further supposes that 

 beneath this doleritic material the quantity of iron-oxide, capable 

 of producing the last named mineral, goes on increasing, and that 

 ultimately a point is reached- from which to the centre metallic 

 elements alone exist. In further reasoning as to the condition of 

 this metallic centre, Von Waltershausen takes into consideration 

 the influence of the superincumbent pressure upon the fusing 

 point of the metals. The following is a translation of his remarks 

 on this subject: " For sometime past Bunsen has devoted his at- 

 u tention to this subject, and described (in Poggendorff's Annalen, 



