114 H. S. JEVONS, H. I. JENSEN AND C. A. SUSSMILCH. 



There is extremely little evidence available as to the 

 mode of origin of abyssal rocks, so that it is impossible to 

 verify or disprove speculations on this subject. Hypotheses 

 may be obtained deductively, however, from certain general 

 principles and well established facts. The extent of their 

 truthfulness is then dependent on the completeness of the 

 facts taken into consideration, and the soundness of the 

 reasoning. Proceeding from our knowledge of the specific 

 gravity of the earth, and the analogy of meteorites, we 

 conclude that the greater part of the interior of the earth 

 consists of uncombined metallic alloys, perhaps chiefly iron. 

 Whilst the earth was still molten, however, the heavier 

 metals must have tended to be aggregated towards the 

 centre, and the lighter elements (such as silicon, aluminium, 

 and the alkalies) towards the surface. As the earth's 

 incandescent atmosphere cooled, the oxygen would probably 

 unite first extensively with silicon, aluminium, sodium and 

 potassium. Either at once, or before the temperature had 

 been much lowered, some of the abundant silica would 

 combine with these other oxides forming the polysilicates, 

 which later crystallised as felspars. Oxygen would be 

 carried to lower strata partly by water vapour, and partly 

 by elements forming more than one oxide. Silica might be 

 conveyed by the poly- and meta-silicates which would be 

 reduced to meta- and orthosilicates by the native metal in 

 presence of water-vapour, sulphur-dioxide, etc., producing 

 an olivine-ijolite or nephelinite magma. This oxidation of 

 the lower metallic stratum, consisting chiefly of iron with 

 magnesia alloyed in it, would be assisted by frequent dis- 

 turbances due to rapid contraction of the half formed crust. 

 Thus a granite magma might be mixed with an olivine- 

 theralite magma, and produce a gabbro or essexite magma. 

 I believe, however, that the vast predominance in quantity 

 of acid rocks such as granites and granitic gneisses, schists, 

 etc., over more basic rocks in the cores of old mountain 



