320 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



352 to 518 miles, substances may exist having a density 

 of from 4. to 5. ; such as magnetic iron ore, ilmenite, copper, 

 iron, and magnetic pyrites, variegated copper ore, sulphuret of 

 antimony, and perhaps antimonio-sulphurets. From 518 to 705 

 miles in depth, substances may be present having a specific gravity 

 of from 5. to 6. such as iron pyrites, millerite, and copper glance. 

 Deeper still, and until a depth of 923 miles, a density of from 



6. to 7. may be supposed to exist, and consequently arsenio- 

 sulphurets of iron, cobalt and nickel, such as arsenical pyrites and 

 speiss-cobalt, cobalt glance, and tesseral pyrites to be present. 

 Between this depth of 923 miles, and that of 1187 miles, where 

 according to the calculation already mentioned, the surface of the 

 metallic globe may be found, we may suppose a density of from 



7. to 8. to exist, and more or less pure arseniurets, such as the 

 purest speiss-cobalt, arseniurets of copper and nickel, &c, to be pre- 

 sent. It will be evident that in calculating the results above given, 

 I have only been endeavouring to develope Von Waltershausen's 

 theory, and in some measure to correct his results. I say correct 

 them, because in one instance assuming the sp. gr. of the surface as 

 2.66, he arrives at the result that the thickness of the earth's crust 

 does not exceed 67 English geographical miles. So far as regards 

 the composition of the various concentric layers deduced from 

 their specific gravities, I may remark that I have observed a 

 similar succession to that above indicated, manifest itself in 

 smelting cobalt ores. This operation is carried on at Modum in 

 Norway, where on drawing the metal from the furnace there are 

 formed in the crucible receiving it, four different layers of material, 

 which from the surface downwards, are as follows, viz. : Slag, 

 containing about 60 per cent, of lime and oxide of iron; 2nd, 

 sulphurets of copper, iron and cobalt ; 3rd, Arseniosulphurets of 

 iron and cobalt, graduating into 4th, impure metallic iron, mal- 

 leable and containing cobalt. The accompanying sketch shows 

 part of a section of the earth, exhibiting the size of the central 

 metallic globe, the thickness of the concentric layers, and of the 

 solid crust according to the above calculations.- As to whether 

 the metallic globe in the centre is in a solid state, there would 

 appear to be good grounds for this supposition, because apart 

 from the consideration that the solidifying point rises with the 

 pressure, it is well known that in many smelting furnaces, me- 

 tallic iron can accumulate in the bottom, while the slag maintains 

 its fluidity and runs perfectly free from the furnace. Mr. Sterry 



