156 ABIOGENIC ORGANIC-CHEMICAL EVOLUTION 



active decay of the actinouranium and of one of the isotopes 

 of potassium present there ; later on, it will have been 

 determined more by that of uranium and thorium. In con- 

 sequence of this, the interior of the Earth became heated, 

 reaching at some points temperatures of the order of 1,000° C 

 or more. At the high pressures which also prevailed, there 

 resulted a redistribution of the substances present — the 

 heavier aggregates, rich in iron, sank inwards, while those 

 that were lighter (silicates) floated towards the surface. This 

 led to the formation of the crust of the Earth, the lithosphere, 

 as a result of the lighter rock formations being squeezed out 

 in a molten state on to the surface of the planet. This process 

 continued throughout geological time and cannot even now 

 be regarded as at an end. 



Intimately linked with the formation of the lithosphere 

 is the development of the hydrosphere and of the primaeval 

 atmosphere of the Earth. ^ The amount of water present on 

 the surface of the Earth was much less than that now present. 

 This was gradually increased by the decomposition of hy- 

 drates and the liberation of water of constitution from the 

 interior of the Earth.''* 



The hydrosphere was also markedly different in its chemi- 

 cal composition. The waters of the primitive seas and oceans 

 were poorer in inorganic salts than are their present-day 

 counterparts. The migration of the elements which make up 

 these salts only proceeded rather slowly, chiefly as a result of 

 the natural circulation of water. This migration was a very 

 important preliminary stage in the development of life. 



The temperature both of the hydrosphere and of the atmo- 

 sphere was largely determined by the radiation reaching the 

 Earth from the Sun. The strength of this seems scarcely to 

 have changed during the whole period in which the Earth 

 has existed. 



The principal qualitative difference from present-day con- 

 ditions was in the composition of the primaeval atmosphere. 

 The atmosphere to-day has an oxidising character, being very 

 rich in free molecular oxygen. But the overwhelming bulk 

 of this gas was formed, and continues to be formed, bio- 

 genically, as a result of the activity of green plants. The total 

 amount of oxygen in the present-day terrestrial atmosphere 



