THE MAKIXG OF THE GEOLOGICAL TIME-SCALE. II 



is known as well as the extent of its unreliability. In early 

 attempts to classify 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. 



Ancient Notions of Geology. — The ancients in many respects 

 were keen observers ; they knew much about plants, animals, 

 physical and chemical phenomena, and astronomy. But, 

 with all their learning, there appears to have been no concep- 

 tion formed of an ancient history of the globe and its inhab- 

 itants 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 mountains by earth- 

 quakes, 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 that 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."* 



Beginnings of a Scientific System of Classification. — By degrees, 

 as Lyell has described in such fascinating manner, one after 

 another the foundation principles were announced, discussed, 

 controverted, and finally, by their intrinsic truth, became estab- 

 lished. But it was not till nearly the beginning of the present 

 century that enough was known of rocks for the formation of 

 a general systematic classification of geological formations. 

 The belief in a limit of six thousand years for the formation 

 of the world was prevalent. Catastrophe was the universal 

 resort for explanation of phenomena not then understood. 

 And for geological purposes the Noachian deluge was an in- 



* Lyell's Principles, p. 34. 



