Geological History of the Atmosphere. 445 



that the metallic iron produced by the heating of the organic 

 or carbonaceous matter present in those deposits along with 

 oxide of iron and other iron compounds also present would 

 sink deep down — possibly as a rule to inaccessible depths. 

 It should also be remembered when discussing the question 

 of the production of the heat necessary for our theory, that 

 the combination of the carbon or organic matter with the 

 oxygen of the iron compounds would probably tend to raise 

 the temperature of the mass to an appreciable extent, and 

 that the long time during which the materials would be 

 heated together, or even the long time during which the iron 

 compounds would be heated in the presence of reducing 

 gases derived from the destructive distillation of the organic 

 matter, would be in favour of a more thorough reduction of 

 the iron compounds to the metallic condition than we should 

 expect to take place if the very same materials were heated for 

 a short time in a modern blast-furnace. Another point worth 

 noting is that the reduction of oxygenated iron compounds by 

 carbonaceous matter may have taken place, at all epochs and 

 on a very large scale, in the presence of sulphur compounds 

 at quite ordinary temperatures, and it is also possible that a 

 large quantity of the sulphide of iron produced in this way 

 may have sunk down to an inaccessible depth when the 

 rocks containing it were heated to fusion by volcanic action 

 (or the causes which bring about volcanic action). Of course 

 the disappearance of sulphide of iron in this way involves the 

 question of heating up just as much as reduction to the 

 metallic state would, but still enough has already been said 

 to show that the question of heating does not, or at least 

 need not, put insuperable difficulties in the way of our theory. 

 Questions regarding the nature of the vegetation or organic 

 life in those very early epochs need not cause any serious 

 difficulty, for organisms of the humblest type, such as 

 diatoms or infusoria, would be sufficient for the purpose. The 

 very obvious difficulty regarding the absence or comparative 

 absence of light at the base of the great atmosphere which 

 we have conjectured, and the probably adverse conditions for 

 the growth of vegetation thereby produced, may be met by 

 a -king the question — Why do plants require light to promote 

 their growth? Is it not to decompose carbonic acid and 

 water in order to produce compounds containing a smaller 

 proportion of oxygen ? If compounds containing less oxygen 

 were already present in the atmosphere and ocean in abund- 

 ance, the conditions would be very different, and in these 

 conditions it is quite possible that vegetable growth or organic 

 growth of some kind might go on in the absence of light. 



