636 PHYSIOGRAPHY 



tice, is that it may tend toward empty ritualism. To give the idealized 

 types chosen for convenience of classification an appropriate atmo- 

 sphere, the fact that changes are constantly in progress that moun- 

 tains come and go even as the clouds of the air form and re-form 

 should be ever present in the mind. 



The process of evolution without concurrent extinction which 

 characterizes the development of physiographic features finds expres- 

 sion also in related departments of nature, as, for example, in petro- 

 graphy, where, as is well known, it has greatly delayed the framing of 

 a serviceable and logical system of classification. Indeed, the princi- 

 ple referred to may be said to be one of the chief distinctions between 

 the organic and the inorganic kingdoms of nature. 



The Primary Features of the Earth's Surface 



The primary features of the earth's surface may consistently be 

 defined as those resulting from the growth and internal changes of 

 the lithosphere, while the modifications of relief resulting from the 

 action of agencies which derive their energy from without the earth 

 may be termed secondary features. The primary or major character- 

 istics of the earth's surface, so far as now known, may be ranked 

 in three groups, in accord with the agency by which they were princi- 

 pally produced; namely, diastrophic, plutonic, and volcanic physio- 

 graphic features. Each of the groups presents many as yet unsolved 

 problems. 



Diastrophic Features. Under this perhaps unwelcome term are 

 included a large class of elevations, depressions, corrugations, faults, 

 etc., in the surface portion of the lithosphere due to movements within 

 its mass. The causes of the changes which produced these results 

 are as yet obscure, and, although a fruitful source of more or less 

 romantic hypotheses, may in general terms be referred to the effects 

 of the cooling and consequent shrinking of a heated globe, or, under 

 the terms of the planetesimal hpyothesis, reckoned in part among 

 the results of gravitational condensation. However obscure the 

 fundamental cause, the results in view are real, and among the larger 

 of the earth's features with which the physiographer deals. They 

 are the greater of the quarry-blocks, so to speak, which have been 

 wrought by denuding agencies into an infinite variety of sculptured 

 forms. Included in the list, as the evidence in hand seems to indicate, 

 are continental platforms, oceanic basins, corrugated and block 

 mountains, and many less mighty elements in the marvelously varied 

 surface of the lithosphere. Not only the study of the shapes of these 

 features of the earth's surface, but the movements they are still experi- 

 encing, and their transformations through the action of denuding 



