REDUCING CONDITIONS 159 



sphere were now taken up by incompletely oxidised sub- 

 stances. This completely prevented any accumulation of 

 oxygen in the atmosphere of the Earth before life had 

 appeared. Even now, when the reserves of free oxygen in 

 the atmosphere are continually being replenished by green 

 plants, it is only the outermost skin of the crust of the 

 Earth which is oxidised. The deeper formations remained in a 

 strongly reduced state, combining avidly with oxygen. This 

 may be illustrated by the well-known fact that lava and basalt 

 are black, green and grey, showing that they contain iron 

 in an incompletely oxidised state. The sedimentary forma- 

 tions such as clays and sands, on the other hand, are red or 

 yello^v in colour. In these the iron is fully oxidised. Thus, 

 oxygen is gradually being taken up before our very eyes in 

 the transformation of igneous into sedimentary formations 

 and it is only the process of photosynthesis which continually 

 replenishes the atmosphere ^vith this gas. According to the 

 calculations of V. M. Goldschmidt,^^ if all the plants on the 

 Earth were suddenly destroyed the free oxygen of the atmo- 

 sphere would disappear within a iew thousands of years, 

 a very short time on the geological scale ; it would be taken 

 up by incompletely oxidised minerals. 



However, even in such a case, the Earth, though bereft of 

 life, would not return to its original state. The oxidised 

 conditions brought into being by life would leave indelible 

 traces on its surface in the shape of oxidised rock formations. 

 This applies particularly to carbon compounds. Under the 

 reduced conditions prevailing on the primaeval Earth carbon 

 existed mainly in the form of carbides, graphite and hydro- 

 carbons. 



The appearance of free oxygen created the conditions 

 under which hydrocarbons could be oxidised. The final stage 

 in this process was the formation of carbon dioxide, but this 

 could not accumulate in significant amounts in the atmo- 

 sphere because it reacted with the silicates of the lithosphere 

 and was held there as carbonates" in accordance with such 

 an equation as MgSiOo, + cOo->MgC03 + siOo. 



The process of the formation of carbonates was greatly 

 intensified after the appearance of life, and the crust of the 

 Earth now contains enormous deposits of carbonate-contain- 



