THE GEOLOGICAL TIME- SCALE. l8l 



rocks the chronological element of the scale was not considered, 

 but by degrees the classification has passed from a classification 

 of rocks to a classification of periods of time. 



The ancients in many respects were keen observers ; they 

 knew much about plants, animals, physical and chemical phenom- 

 ena, and astronomy. But with all their learning, there appears 

 to have been no conception formed of an ancient history of the 

 globe and its inhabitants prior to the earlier centuries of the 

 Christian era. One of the first geological phenomena to become 

 generalized into a theory was that of the formation of moun- 

 tains by earthquakes, as cited by Avicenus in the tenth century. 

 The gradual change of relative level of land and sea, as seen in 

 the encroaching of the sea or the departure of sea from the 

 shore, gave rise to speculations regarding the great length of 

 time required for the lifting of the whole land by this means. 

 In the sixteenth century, Lyell reminds us, attention was drawn 

 to the meaning of fossils, and dispute arose as to their nature. 

 Leonardo da Vinci doubted the then current belief that the stars 

 were the cause of the fossil shells and pebbles on the mountain 

 sides, and advanced the idea "that the mud of rivers has 

 covered and penetrated into the interior of fossil shells at the 

 time when these were still at the bottom of the sea near the 

 coast" (Lyell's Principles, p. 34). 



By degrees, as Lyell has described in such fascinating man- 

 ner, one after another the foundation principles arose, were dis- 

 cussed, controverted, and finally, by their intrinsic truth, became 

 established. But it was not till nearly the beginning of the 

 present century that enough was known of rocks for the forma- 

 tion of a general systematic classification of geological forma- 

 tions. The belief in a limit of six thousand years for the forma- 

 tion of the world was prevalent. Catastrophy was the universal 

 resort for explanation of phenomena not then understood. And 

 for geological purposes the Noachian deluge was an indispensa- 

 ble agent for the scientific explanation of even the conspicuous 

 phenomena. For these reasons inquiry did not reach into the 

 antiquity of the geological ages. And the first attempts at classi- 



