PHYSIOGRAPHIC PROBLEMS OF TO-DAY 643 



basins, corrugated mountains, block mountains, domes of various 

 and some of vast dimensions upraised by intrusions, volcanic cones, 

 lava-plateaus, etc. These are the major physiographic types, or the 

 larger monoliths from which the rock-hewn temples of the earth have 

 been sculptured by forces acting on the surface of the lithosphere and 

 deriving their energy mainly from the sun. Resulting from surface 

 changes come a vast array of both constructive and destructive 

 physiographic features, which may consistently be termed secondary. 

 Under secondary features may be included also relational topographic 

 forms, such as islands in water, in glaciers, and in lava-fields. In 

 the study of the primary features of the earth's surface the work of 

 the physiographer is most intimately linked with that of the geo- 

 logist, but, on passing to the secondary feature, the influence of life 

 becomes apparent, and the relation of man to nature is in the end the 

 leading theme. 



Secondary Physiographic Features 



The most familiar features of land areas, as is well known, are 

 those which owe their existence to the work of moving agencies 

 resident on the earth's surface, such as the wind, streams, glaciers, 

 waves, currents, etc. The forces at work are set in motion by energy 

 derived from without the earth, and the material worked upon is 

 brought within the range of their activities by forces resident within 

 the earth which cause deformations of, or additions to, its surface. 

 The earth-born primary physiographic features are thus modified 

 by sun-derived forces, and a vast array of secondary modifications of 

 relief are produced which give variety and beauty, particularly to 

 those portions of the lithosphere which are exposed for a time to the 

 air. The study of secondary physiographic features has produced 

 a rich and abundant harvest, especially during the last quarter of a 

 century, and the returns are still coming in at a seemingly accel- 

 erated rate. 



The themes for study are here mainly the various processes of 

 erosion and deposition of the material forming the outer film of the 

 lithosphere, and the characteristics of the destructive and construct- 

 ive topographic forms produced. With the knowledge gained con- 

 cerning the changes now in progress on the ocean's shore, in the 

 forest, by the riverside, on the snow-clad and glacier-covered moun- 

 tains, etc., the physiographer seeks to decipher the records made in 

 similar situations during the past. Two groups of problems are in 

 sight in this connection; one is concerned with observing, classifying, 

 and recording the changes now in progress; and the other has for 

 its chief aim the translation, in terms of the agencies now at work, 

 of the records left by past changes. We find that to-day the same 



