PHYSIOGRAPHIC PROBLEMS OF TO-DAY 641 



ive methods for considering 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 of as 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 fissure or regional eruptions are rendered still more numerous 

 by the suggestion 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 meas- 

 ures 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 earth's atmosphere, as well as the changes it is now 

 undergoing, is a theme in which the physiographer is profoundly 

 interested. 



Volcanic mountains are numbered among the most awe-inspiring 

 of topographic forms; the solid additions which volcanoes make to 

 the surface of the lithosphere are in view, and the contributions to 

 the atmosphere of vapors and gases from the same sources are tang- 

 ible facts; but another phase of the great problem is also of interest 

 to the physiographer, namely, what changes take place in the rigid 

 outer shell of the earth by reason of such transfers of vast volumes 

 of material as are known to have occurred from deep within the earth 

 to its surface. The magmas which have been caused to migrate and 

 come to rest for a time, either as intrusions within the earth's outer 

 shell, or as extrusions on its surface, are measurable in millions of 

 cubic miles. In connection with the profound questions concerning 

 the formation of folds and fractures in the earth's crust, an agency 

 is thus suggested comparable in importance with loss of heat, as 

 under the nebular hypothesis, or with gravitational compression, as 

 explained by the planetesimal hypothesis. In the many discussions 

 that have appeared as to the adequacy of earth contraction to 



