540 ISRAEL C. RUSSELL 



extended and more careful search. The progress of physiography 

 would evidently be accelerated by a systematic review and a more 

 definite classification of the topographic forms, both constructional 

 and destructional, known to have resulted from volcanic agencies, 

 and a more critical selection of types to serve as species than has as 

 yet been attempted. From such a catalogue something of the under- 

 lying principles governing the many ways in which the relief of the 

 earth's surface has been modified, and is still being changed through 

 .the agency of volcanoes, -would make themselves manifest, and pre- 

 dictions rendered possible which would facilitate further study. The 

 analogy between lava streams and rivers, on the one hand, and glaciers, 

 on the other, suggests interesting and instructive methods for con- 

 sidering the entire question of the movements of liquids and solids 

 on the earth's surface. 



While the topographic changes produced by volcanic agencies 

 are of chief interest to the physiographer, they lead him to profound 

 speculations in reference to the nature of the forces to which they are 

 due,' the source and previous condition of the matter extruded during 

 eruptions, and the study of the existing relations between the earth's 

 interior and its surface. The great, and as yet but partially answered, 

 questions : Whence the heat manifest during volcanic eruptions ? 

 What is the source of the energy which forces lava to rise from deep 

 within the earth through volcanic conduits to where it is added to the 

 surface, perhaps ten to twenty thousand feet above sea-level ? and, 

 what is the source or sources of the steam discharged in such vast 

 quantities during eruptions of even minor intensity ? are as of great 

 interest to the physiographer as they are to the geologist, and furnish 

 another illustration of the unity of nature-study. From the new point 

 of view furnished by the author of the planetesimal hypothesis, the 

 many questions the physiographer is asking concerning volcanoes 

 and fissui'^e eruptions are rendered still more numerous by the sugges- 

 tion that these fiery fountains are the sources from which the ocean 

 and all the surface waters of the earth have been supplied. This 

 startling revelation, as it seems, makes a still more urgent demand 

 than had previously been felt for quantitative measures of the vapor 

 discharged from volcanic vents. Nor is this all; with the steam of 

 volcanoes is mingled various gases, and the mode of origin of the 



