640 PHYSIOGRAPHY 



plugs, will demand a place in the group of type examples of primary 

 unsculptured elements in the relief of the lithosphere. 



Volcanic Features. Elevations on the surface of the lithosphere 

 due to the presence of material extruded from volcanic vents have 

 long been recognized, but the specific, or, as perhaps may be con- 

 sistently claimed, generic, differences among them have only recently 

 claimed attention. Of primary importance in the classification of 

 topographic forms of volcanic origin is the fact that volcanoes are 

 both constructive and destructive in their action. Among the results 

 of constructive action are included the changes produced by effusive, 

 fragmental-solid, and massive-solid eruptions, each of which has 

 furnished a wide range of primary topographic forms. The catalogue 

 of recognized types includes lava plains and plateaus, cinder and 

 lapilli cones, lava cones and domes, lapilli and dust plains, together 

 with many minor structures, such as "spatter-cones," "lava-deltas," 

 "lava-gutters," "lava-levees," and the various surface details of 

 lava-streams due to the flow of still mobile magmas beneath a stif- 

 fened crust which ranged in physical consistency from a highly plastic 

 to a rigid and brittle condition. With these more familiar forms are 

 to be included also the results of massive-solid extrusions, of which 

 the "obelisks" of Mont Pelee are the most striking examples. 



Our present list of destructional topographic forms due to volcanic 

 eruptions includes decapitated cinder, lapilli, and lava cones, and 

 subsided and broken lava-domes, calderas, crater-rings, etc., together 

 with cones of various kinds breached by outflowing lavas; and, as 

 minor features, the floated blocks sometimes carried on lava-streams, 

 or the moraines of lava flows, as they may suggestively be termed, the 

 subsided and broken roofs of lava-tunnels, etc. 



The interesting contributions made during the past decade to the 

 list of topographic forms resulting from the action of volcanic 

 agencies are highly suggestive, and warrant the belief that still more 

 numerous and equally important results in the same direction will 

 reward more 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 con- 

 structional 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 underlying 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, w r ould make themselves 

 manifest, and predictions 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 instruct- 



