44 EVOLUTION OF THE EARTH 



ened. With the separation of the lands from the seas, ero- 

 sion began, carbon dioxide was abstracted from the atmosphere 

 to make carbonates, and a further cause of atmospheric 

 depletion was initiated. Thinner, rarer, and colder grew the 

 gaseous envelope, until an oscillating balance was established 

 between the supplies of new gases from the uprising molten 

 rocks and the losses involved in the weathering of their solid 

 forms. Nitrogen was at first relatively small in quantity and 

 oxygen not present in more than a trace. An evolution in 

 atmospheric composition had still to go forward through 

 following ages to transform it into a gaseous medium for the 

 support of the higher life. But even in the early periods 

 following the gathering of the oceans and the emergence of the 

 lands, the sun warmed the atmosphere and earth. An environ- 

 ment suitable for the lowest organisms had arisen and the 

 earliest forms of life may not have been long in coming into 

 existence. The reign of the surface processes had begun, but, 

 at age-long intervals, the still youthful energies of the interior 

 broke forth. Magmas in great volume ascended, now seen as 

 the most ancient granite-gneisses. In the great crustal over- 

 turning of these earliest revolutions the foundation rocks ap- 

 pear to have been everywhere destroyed. The oldest rocks pre- 

 served are mashed and crystallized sediments and lava sheets 

 resting as fragments of a cover on the reservoirs of younger 

 magma. Such sediments, altered and intruded, are the 

 oldest Archean rocks: It is not known how close they stand 

 in point of time to the formative processes whose description 

 has been attempted. With these oldest rocks, the dimly 

 known, heroic and mythical eon of the earth is closed and the 

 first historic eon opens as the remote and long enduring 

 Archean division of geologic time. 



