34 EVOLUTION OF THE EARTH 



makes familiar the fact that slag is insoluble in iron and, 

 being lighter, gathers in the upper part of the crucible, like 

 cream upon milk. The slag is similar in_j^Qmposition to 

 basaltic igneous rocks. The density of the deep interior sug- 

 gests that it is layered like the crucible of the blast furnace 

 and that the silicate rocks form an envelope some hundreds 

 of miles thick, grading down into a great metallic core. The 

 silicate envelope ultimately differentiated further, resulting 

 in a rise of the more siliceous and lighter fraction into an outer 

 layer, perhaps 50 to 75 miles in thickness. This in turn crys- 

 tallized into a primordial, universal, granitic crust above a 

 thicker basaltic shell below. 



The primordial atmosphere. Granting the conclusions in 

 regard to the initial fluid state of the earth, let the primordial 

 atmospheric conditions be pictured. A gaseous envelope exist- 

 ing in equilibrium with rock magma would be dominantly 

 water-vapor, followed in order of importance by carbon 

 dioxide and carbon monoxide, chlorine and hydrochloric acid, 

 with some nitrogen, but no free oxygen. 



The present atmosphere of nitrogen and oxygen penetrates 

 by solution into the liquid envelope of water. The primitive 

 atmosphere of water-gas penetrated in the same manner by 

 solution into the ocean of molten rock. But such an admixture 

 of molten rock and water-gas is known to have a remarkable 

 effect upon the melting point of silicate magmas. Under dry 

 fusion the exalted temperature of 1300 to 1500 C., a 

 dazzling white heat, is necessary to make granite thinly molten. 

 But if the fluid rock has dissolved an abundance of water- 

 vapor the mixture stays fluid until it has cooled below a tem- 

 perature of 800 C. The surface of the earth when molten 

 was perhaps no hotter than this. At such a comparatively 

 low temperature and even at somewhat higher temperatures 

 there would be but little dissociation of water into its com- 

 ponent gases, and the earth would be capable of holding to 



