530 ISRAEL C. RUSSELL 



the only portions of our field of study that demand attention. New 

 ideas, new principles, and fresh hypotheses make an unknown country 

 of the most familiar landscape. The definite formulation of the base- 

 level idea, the suggestive and far-reaching principle embraced in the 

 term "geographic cycle-," the planetesimal hypothesis as to the origin 

 of the earth, etc., furnish new and commanding points of view, or, as 

 they may be termed, primary stations in the physiographic survey of 

 the earth's surface by means of which previous local surveys may be 

 correlated and corrected. 



In the search for problems, the unraveling of which may be expected 

 to advance the scientific study of the earth's surface, the writings of 

 travelers, the pregnant suggestions of those who make reconnaissances 

 into the realm of unknown facts and of unrecognized ideas, as well as 

 the precise and accurate pictures of portions of the earth's surface 

 presented on the maps of the topographer and the charts of the ocean- 

 ographer, point the way to still greater advancements, and furnish 

 inspiration to those who follow. 



FUNDAMENTAL CONCEPTS. 



While physiography deals with the surface features of the earth, 

 the fact that in those features is expressed to a great extent the effects 

 of movements originating deep within the earth, leads the student of 

 continents and oceans to ask of the geologist and the physicist puzzling 

 questions as to the changes that are taking place in the great central 

 mass of our planet, and even in reference to the origin of the earth 

 itself. So intimately are the various threads of nature-study inter- 

 woven that the full significance of many of the surface features of the 

 earth cannot be grasped and their genesis explained until the nature 

 and mode of action of the forces within the earth which produce 

 surface changes are understood. 



The growth of physiography up to the present time has been 

 largely influenced by the far-reaching ideas of Laplace and others in 

 reference to the nebular origin of the solar system. In all of the 

 questions respecting secular changes of land areas in reference to the 

 surface level of the ocean, the origin of corrugated and of block 

 mountains, the fundamental nature of volcanoes, etc., the controlling 

 idea has been that the earth has cooled from a state of fusion, and is 



