250 PROGRESS OF SCIENCE IN THE CENTURY. 



tion of the value of fossils. To that source may be 

 traced the prodigious development of stratigraphy over 

 the whole world, the power of working out the geologi- 

 cal history of a country, and of comparing it with the 

 history of other countries, the possibility of tracing the 

 synchronism and the sequence of the earth's surface 

 since life lirst appeared upon the planet." * 



PROBLEMS OF EARTII-SCULPTURE. 



What is often called " Dynamical Geolog}^ " 

 is concerned v^^ith the factors which have wrought 

 out the present state of the various land-forms. It 

 has to do with the evolution of scenery, or with 

 earth-sculpture, — one of the most fascinating prob- 

 lems of geology. 



Air, water, ice, volcanoes, earthquakes, changes 

 in coast-level, thrust-movements, living creatures, 

 are the most important factors in the process by 

 which the face of the earth has been and is being 

 slowly changed. To some of these we wish to refer 

 in this section, while others have found notice else- 

 where. 



JIiiMons Recognition of Factors in Earth-Sculp- 

 ture. — In his Theory of the Earth (1788), Hutton 

 recognised the following factors as operative in 

 changing the earth's surface: — degradation of land 

 by atmospheric and aqueous agencies, deposition of 

 the debris as sediment in the ocean, consolidation and 

 metamorphosis of sedimentary deposits by the in- 

 ternal heat and by injection of molten rock, disturb- 

 ance and upheaval of oceanic deposits, and forma- 

 tion of rocks by the consolidation of molten material 

 both at the surface and in the interior of the earth. 



* Sir A. Geikie. Founders of Geology, 1897, p. 241. 



