PHYSIOGRAPHIC PROBLEMS OF TODAY 539 



In our dreamed-of museum of ideal physiographic types, mighty 

 domes raised by regional intrusions, broad uplifts with perhaps 

 sharply defined boundaries, elevated by relatively thin intruded 

 sheets, as well as steep-sided domes with relatively small bases, con- 

 cealing laccoliths, and the still smaller covers arching over plutonic 

 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 has 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 stiffened 

 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 "obehsks" of Mont Pele 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 0} 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 behef that stifl more numerous 

 and equally important results in the same direction will reward more 



