ill THE RENASCENCE OF SCIENCE 93 



philosophers 'come to their own' again. Xenophanes 

 of Colophon has been referred to as arriving, five 

 centuries B.C., at a true explanation of the imprints 

 of plants and animals in rocks. Pythagoras, who 

 lived before him, may, if Ovid, writing near the 

 Christian era, is to be trusted, have reached some 

 sound conclusions about the action of water in the 

 changes of land and sea areas. But we are on surer 

 ground when we meet the geographer Strabo, who 

 lived in the reign of Augustus. Describing the 

 countries in which he travelled, he notes their 

 various features, and explains the causes of earth- 

 quakes and allied phenomena. Then eleven hundred 

 years pass before we find any explanation of like 

 rational character supplied. This was furnished by 

 the Arabian philosopher, Avicenna, whose theory of 

 the origin of mountains is the more marvellous when 

 we remember what intellectual darkness surrounded 

 him. He says that ' mountains may be due to two 

 different causes. Either they are effects of up- 

 heavals of the crust of the earth, such as might occur 

 during a violent earthquake, or they are the effect 

 of water, which, cutting for itself a new route, has 

 denuded the valleys, the strata being of different 

 kinds, some soft, some hard. The winds and waters 

 disintegrate the one, but leave the other intact. 

 Most of the eminences of the earth have had this 

 latter origin. It would require a long period of time 

 for all such changes to be accomplished, during 

 which the mountains themselves might be somewhat 

 diminished in size. But that water has been the 

 main cause of these effects is proved by the existence 

 of fossil remains of aquatic and other animals on 



