10 Causes and Course of Organic Evolution 



typical samples of the primitive part of the earth's crust; and 

 from this it will be seen that the group of oxides given makes 

 up about 99 per cent, of its total mass. 



Silica (SiO,) 59.71 



Alumina (AI2O3) 15 . 41 



Ferric oxide (FegOg) 2. 63 



Ferrous oxide (FeO) 3.52 



Calcic oxide (CaO) 4 . 90 



Magnesia (MgO) 4.36 



Potash (K2O) 2. 80 



Soda (NagO) 3. 55 



Water (H2O) 1.52 



Phosphoric acid (P2O5) 0. 22 



98.62 



It is generally conceded that, during the early formation 

 of this shell or crust, abundant and widespread volcanic activity 

 on the one hand combined with increasing cooling of the crust 

 on the other were proceeding, so that, with evolution of a 

 definite enveloping atmosphere, subaerial denudation would, 

 on the one hand, cause steady attrition, disintegration, and 

 removal of the previously formed masses, wliile new accumula- 

 tions of molten material, thrown out from the earth's interior, 

 would cause, by gradual coohng of these, steady upbuilding 

 or integration of the crust. 



At the close then of what may be termed the igneous or 

 volcanic epoch of the earth's history, the internal masses of 

 gaseous iron and rarer metals, along with smaller quantities 

 of other elements that now formed, on Arrhenius's view, the 

 earth's core, were bounded by a transition layer of viscous and 

 unstable material, that again was enveloped by a granite shell. 

 Or to picture it from "wdthout inward in Geikie's words: ''The 

 interior of the earth, therefore, with the exception of a solid 

 crust about 40 kilos tliick, consists of a molten magma 100 to 

 200 kilos in depth, which shades continually inward into a 

 gaseous center. The liquids and gases in the interior possess 

 a viscosity and incompressibility such as permit them to be 

 regarded as solid bodies." 



