INTRODUCTION. 23 



periods of the earth's history by means of an ordered array of 

 extinct fossil forms. 



Hypotheses of the Earth's Origin and history, and 

 Beginnings of Geological Observation. The ^een interest in 

 minerals and fossils and the flourishing condition of the 

 mining industry gradually attracted the attention of scientific 

 men to the investigation of the earth itself. Two methods 

 of research, the empirical and the speculative, developed 

 alongside one another. The one had for its immediate aim 

 the determination of facts, and in its further outlook, the 

 possible construction of some suitable theory ; the other 

 contented itself with a minimum of observation, accepted the 

 risks of error, and set about explaining the past and the 

 present from the subjective standpoint. This latter method 

 naturally attained no higher results than the geogenetic 

 fantasies of classical antiquity. And it certainly could never 

 have gathered sufficient energy to roll aside the mass of 

 philosophical and doctrinal tradition that blocked the path of 

 progress. 



Throughout the later and Middle Ages, water and fire still 

 continued to be accepted as the two essential active and 

 formative forces dominating the earth's configuration, hence 

 it was unavoidable that the conceptions of the ancient philo- 

 sophers should re-appear again and again in the newer theories, 

 if in renovated form. Meantime there were in every land of 

 Europe empiricists who were patiently contributing new data 

 to the knowledge of chemistry, of physics, and the constitu- 

 tion of the earth's crust, and were thus preparing the only 

 possible foundation of a science of geology. 



Leonardo da Vinci deserves an honoured place amongst the 

 founders of geology, as one of the first who investigated the 

 earth's structure upon scientific principles. Not only did 

 Da Vinci recognise the true origin of fossils, but his artistic 

 sense of form and his close observation of nature revealed to 

 him in the North Italian valleys the agency of running water 

 in sculpturing the earth's surface. He showed how rivers erode 

 their valleys, and deposit pebbles on valley terraces; how a 

 fine detritus accumulates at river mouths, and plants and 

 animals are buried in it ; how the organic remains then pass 

 through physical changes and become petrified while the 

 river mud hardens into solid rock, and finally the rock 



