542 ISRAEL C. RUSSELL 



are they, to be asked of volcanoes, whether active, dormant, or dead, 

 and in various stages of decay and dissolution, it is plain that all 

 the facts that can be learned concerning them should be classified and 

 put on record, and their more obvious bearings on the fundamental 

 questions concerning the condition of the earth's interior, and the 

 changes there taking place, pointed out. In this connection — and 

 as is true in all branches of research — the fact may be recalled that 

 energy expended in discovering, classifying, and recording facts 

 decreases the time and force necessary for the framing of multiple 

 hypotheses. With an abundance of well-classified and pertinent 

 observations in hand, the nature of the thread on which the gems 

 of truth should be strung usually declares itself. 



Resume. — On a previous page of this essay the desirability was 

 suggested of recognizing ideal types with the aid of which the multi- 

 tudinous surface features of the earth could be classified and studied. 

 Thus far we have considered the elements in the relief of the earth's 

 surface which have resulted from changes within its mass. We term 

 them primary physiographic features, because their birth precedes 

 the modifications of the lithosphere due to agencies acting externally. 

 They are (i) the topographic forms resulting from contraction on 

 account of coohng, or of condensation owing to growth in mass; (2) 

 the surface changes produced by intrusions of magmas into the 

 earth's outer shell; and (3) the results of volcanic eruption. Among 

 the more important idealized models in our future physiographic 

 museum there should be displayed continental platforms, oceanic 

 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 geolo- 



