438 Mr. J. Stevenson on the Chemical and 



atmosphere. In any case we must admit that the large 

 quantities of sesquioxide of iron and of sulphates found on 

 the earth form a very formidable counterpoise to the car- 

 bonaceous matter, and therefore weaken to a very considerable 

 extent the position taken up in my first paper regarding the 

 history of free oxygen. But on the other hand there are 

 •certain considerations which tend to strengthen the position 

 very materially. It is quite possible that in the treatment of 

 this question allowance should be made for carbonaceous 

 matter which formerly existed on the earth, but which has 

 been destroyed in the course of geological history, for not- 

 withstanding its destruction, its former existence and the 

 mode of its destruction might have an important bearing on 

 the question before us. 



It is obvious that coal and similar carbonaceous matters 

 would undergo great changes if the strata containing them 

 should become highly heated, and that they would even be 

 destroyed or disappear if they should be highly heated in 

 contact with oxide of iron and certain other metallic oxides. 

 In that event the carbon would be converted into carbonic 

 oxide or carbonic acid, and the metallic oxides would be 

 reduced to the metallic condition, or at any rate to compounds 

 containing a smaller proportion of oxygen than before. At 

 first glance one would be inclined to say that this reaction 

 cannot have taken place to any great extent in the course of 

 geological history on account of the comparative rarity of 

 native metallic iron and other free metals, but a little reflection 

 will show that the reaction may quite possibly have taken place 

 on a large scale in very early times, though little or none of 

 the metallic iron then produced may be now visible. It is 

 obvious that when the rocks became heated up to the point 

 at which the metallic oxides present were reduced to the 

 metallic condition the rocks would either be fused or would 

 be not far from the fusing-point, and if reduction should 

 sometimes have taken place before fusion, still the process of 

 heating would probably, in most cases, be continued until 

 fusion also took place, and in that case the metallic iron or 

 other metals produced would sink down through the melted 

 rock-mass. If this melted mass should be very great or 

 deeply-seated within the crust of the earth, the metal might 

 sink to such a depth that it would never afterwards be 

 brought by geological changes sufficiently near the surface 

 of the earth to come within the range of human observation. 



Fortunately for the purposes of this inquiry there are 

 sufficiently substantial data from which to make some 

 inferences as to the probability of the reaction having taken 



