IGNEOUS INTRUSIONS AND VOLCANOES. 241 



tased largely on what is seen when one looks down into the throat 

 of a volcano in a state of mild activity ; let us supplement such a 

 view by endeavoring to form a concej)tion of the conditions that 

 exist far below the surface. 



In many instances volcanoes are known to be situated on lines 

 of fracture in the earth's crust. In all volcanoes it is evident that 

 there is a passageway or conduit, leading from an intensely heated 

 region within the earth, to the surface. These conduits must be 

 several thousand feet in depth. Indeed, it is not unreasonable to 

 assume that they may have a depth of several miles or possibly 

 tens of miles. What one sees, therefore, in looking into a crater 

 of an active volcano is the summit of a column of molten rock, 

 the bottom of which is tens of thousands of feet below. 



Judd has compared the mild activity of Stromboli to the boil- 

 ing of mush in a tall vessel, the heat being applied at the bottom. 

 Steam is generated in the mush, and, rising through it in bub- 

 bles, elevates the surface. When the bubbles of steam burst, 

 portions of the viscid material are blown into the air. Such an 

 analogy is certainly sustained by what is seen at the summit of a 

 column of molten lava when we look into the crater of a volcano. 



In seeking for information concerning the conditions that exist 

 far below the surface, when a volcano is giving off steam at the 

 top, we may obtain a few facts to guide us by studying the ruins 

 of extinct volcanoes and the nature of igneous intrusions as laid 

 bare by erosion. 



Volcanic necks tell something of the conditions that exist 

 within a volcanic mountain. In more deeply eroded volcanic dis- 

 tricts one finds dikes and intruded sheets. These are connected, 

 in reference to mode of origin, with Plutonic plugs, laccolites, and 

 what I have termed subtuberant mountains.* These various 

 forms taken by intruded rocks and surface extrusions, as I have 

 attempted to show in the article just referred to, belong in a 

 single genetically connected series. A break in the earth's crust 

 which reaches a region of great heat may be injected with plastic 

 rock and form a dike ; if the fracture terminates above in a re- 

 gion of horizontally stratified beds, the magma rising through it 

 may expand widely, at the same time lifting a broad cover to a 

 comparatively small height that is, form an intruded sheet ; or 

 be more restricted in its expansion, according to its degree of 

 fusion, depth below the surface, and possibly other causes, and 

 raise a cover of less diameter to a greater height that is, form 

 a laccolite ; or, if a great volume of plastic material is intruded, 

 give origin to a subtuberant mountain. Should the fissure reach 



* On the Nature of Igneous Intrusions. In The Journal of Geology, Chicago, vol. iv, 

 1896, pp. 177-194. 



VOL. L. 20 



