Geological History of the Atmosphere. 441 



of oxide of iron, and that mostly or very largely in the 

 form of sesquioxide of iron. We may therefore conclude 

 that basic rocks contain on the average about 6 per cent, 

 more of oxide of iron than the more recent acidic rocks. 



Now this difference in composition may quite well have been 

 brought about by the action of carbonaceous matter at a high 

 temperature on basic rocks or on the materials derived from 

 them by ordinary disintegration and denudation. The forma- 

 tion of metallic iron by volcanic or plutonic agency is not 

 only conceivable but is very highly probable. The molten 

 or heated materials lying beneath volcanoes no doubt 

 consist in many cases to a large extent of what had been 

 sedimentary rocks, containing among other things oxide of 

 iron and carbonaceous matter, which, when heated together, 

 would react on each other, forming carbonic acid (or carbonic 

 oxide) and metallic iron. Even if the oxide of iron and the 

 carbonaceous matter should not have been in direct contact 

 with each other at first, they would be brought into contact 

 by the agitation caused in the mass by the evolution of 

 carbonic acid from carbonates, of steam from clays, shales. 

 and other hydrated rocks, and of hydrocarbon gases and 

 carbonic oxide from the carbonaceous matter itself, while in 

 some cases the hydrocarbon gases and carbonic oxide might 

 themselves be able to effect or at least to assist in the 

 reduction of the oxide of iron. The production of metallic 

 iron is therefore just what we might expect as one of the 

 results of volcanic action, and if iron of volcanic origin has 

 not yet been met with, or only to a very insignificant extent, 

 this circumstance may be explained by the sinking of the 

 iron down into inaccessible regions, or by its subsequent 

 oxidation through exposure to the action of atmospheric free 

 oxygen. Native iron in small quantities has been observed 

 in many localities in both Europe and America, especially in 

 granitic and basaltic rocks. Metallic iron has even been 

 known to be produced in France by a fire in a coal-mine, 

 from which we may infer that if the materials existing in 

 that mine were heated up in a volcano, a considerable 

 quantity of metallic iron would be produced. Further, when 

 we reflect that there must be many coal-bearing regions in 

 the world where the percentage of carbonaceous matter over 

 large tracts of country, and down to very considerable depths, 

 amounts to 1 per cent, and upwards, the reasonableness of 

 our standpoint is quite apparent. In the deep bore at 

 Paruschowitz, in Silesia, which is 2003 metres deep, 83 seams 

 of coal, having a total thickness of about 90 metres, have 

 been met with. This makes the thickness of the coal 4' 5 per 



Phil. Man. S. 6. Vol. 4. No. 22. Oct. 1902. 2 G 



