536 ISRAEL C. RUSSELL 



agencies, only a small part of the difficulties to be overcome have been 

 met. The aim in view is the attaining of a knowledge of what would 

 have been the shape and surface features of the sohd earth, had there 

 been no modifications by internal causes except diastrophism, and no 

 changes in relief by erosion or other surface agencies. Included in 

 this branch of physiography is the shape of the earth itself, in the 

 study of which the physiographer became a geodesist. The earth's 

 shape, and its primary surface features due to diastrophism, form the 

 logica^. basis for physiographic study, in which ideal types of topo- 

 graphic forms declare their usefulness. In the geographical museums 

 of the future, at the head of the long series of models of physiographic 

 types illustrating the species, genera, families, etc., of the earth's 

 surface features, should be placed ideal examples of the most typical 

 elements of relief due to diastrophism. 



Physiographers cannot rest content with the study of the shape of 

 the lithosphere and of its surface relief, in which so much of the 

 history of the earth is recorded, and refrain from searching for the 

 deeper meanings these facts suggest, but must have freedom to invade 

 the province of the geologist, the astronomer, the physicist, the 

 chemist, and other subdivisions of the science of the cosmos, in search 

 of truths bearing on his special line of work. This is particularly 

 true in connection with the special department of physiography in 

 hand, in which many of the branches of the river of knowledge have 

 their sources. 



Plutonic jeatures. — Intimately associated with the irregularities 

 of the earth's surface due to a decrease in its volume, and, as our 

 reasoning tells us, dependent primarily on the same cause and at 

 present only partially differentiated from them, are surface elevations 

 and depressions, produced by the migration of portions of the earth's 

 central magma from the deep interior toward or to the surface. A 

 convenient but arbitrary subdivision of the matter forced outward 

 from the earth's interior is in vogue among geologists, and rocks of 

 plutonic and of volcanic origin are recognized. To the physiographer 

 the distinction referred to is more suggestive than it appears from the 

 point of view of the geologist, since the recognition of differences 

 between topographic forms produced by the injection of fluid or 

 plastic magmas into the cooled, rigid outer portion of the earth, and 



