2 EVOLUTION OF THE EARTH 



igneous rocks. Fossils, the time markers of geology, if once 

 existent, have been destroyed, and, as in the dawn of human 

 history, vast periods of time are dimly sensed through the 

 disordered and illegible record. This crystallized and intri- 

 cately distorted series of the oldest terrestrial rocks tells of an 

 earth surface on which air and water played their parts, much 

 as now. But it was a surface repeatedly overwhelmed by out- 

 pourings of basaltic lava on a vaster scale than those of later 

 ages, and the crust was recurrently broken up and engulfed in 

 the floods of rising granitic magmas. Here the geologic 

 record begins, but the nature of its beginning points clearly to 

 the existence of a prehistoric eon. At the farther bounds of 

 this unrecorded time, forever hidden from direct observation, 

 lies the origin of the earth. 



But the mind of man will not be baffled. Since he may not 

 see directly he will see by inference. Convergent lines of 

 evidence derived from various fields of knowledge may be 

 followed part way toward this goal, like those rays perceived 

 through the telescope on the full moon near the margin of its 

 visible hemisphere, which converge toward craters on the side 

 of the moon that no eye shall ever see. 



Leaving the geologic field of evidence, the problem of the 

 origin of the earth may be approached from the astronomic 

 side. The relationships of the earth to the stars and the 

 planets are displayed in the depths of the heavens, and vestiges 

 must there exist of the cosmic conditions which gave birth to 

 our world and the other planets of our system. The forces 

 of nature are found to obey the same laws as far as the tele- 

 scope can penetrate. The spectroscope detects the familiar 

 t chemical elements in distant stars. These instruments give 

 u assurance of the unity of the cosmos, but the diversity of 

 * objects indicates various stages and various types of evolution. 

 Which approaches nearest to that of our solar system? We 

 must be content to study very much larger and therefore unlike 



