THE RENASCENCE OF SCIENCE. jqi 



anes of Colophon has been referred to as arriving, 

 five centuries b. c, at a true explanation of the im- 

 prints 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 coun- 

 tries in which he travelled, he notes their various 

 features, and explains the causes of earthquakes and 

 alHed phenomena. Then eleven hundred years pass 

 before we find any explanation of like rational char- 

 acter supplied. This was furnished by the Arabian 

 philosopher, Avicenna, whose theory of the origin 

 of mountains is the more marvellous when we re- 

 member what intellectual darkness surrounded him. 

 He says that " mountains may be due to two differ- 

 ent causes. Either they are effects of upheavals 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 emi- 

 nences of the earth have had this fatten origin. It 

 would require a long period of time for all such 

 changes to be accomplished, during which the moun- 

 tains themselves might be somewhat diminished in 

 size. But that water has been the main cause of 



