DALY. — THE NATURE OF VOLCANIC ACTION. 69 



juvenile gases from an underljnng abyssal injection. Entering the 

 crack under pressure and therefore at high temperature, these gases 

 must tend to enlarge it by slow fusion of the wall-rock. The pro- 

 cess may or may not be supplemented by the opening of an explosion 

 funnel at the surface. As the vent is enlarged by gas-flu.xing the 

 magma rises within it, and, kept tiuid by the emanating gas, permits of 

 further upward blowpiping. 



This mechanism implies that the original surface fissure may not be 

 discernilile by the geologist. It may correspond to no vertical or hori- 

 zontal disi)lacement, and at the surface itself be no wider than an ordi- 

 nary master joint or fault fracture. Enlarging slowly downward, such 

 a fissure might be charged with accumulating gases so far as ultimately 

 to cause an explosion. Since the gases must tend to accumulate about 

 one or more points along the fissure, the explosion form will be that of 

 a vertical tube surmounted by a funnel. The resulting vent is a dia- 

 treme, the formation of which was so successfully imitated by Daubree. 

 This type of diatremes is, then, located on a surface fissure, which may 

 or may not be continuous with the abyssal fissure of the primary in- 

 jection. These considerations show the difiiculty of disproving the ex- 

 istence of through-going crustal fissures beneath central vents. 



JJiiitremeK. — Daubree's experiments suggest, however, that some 

 volcanic diatremes may be formed in homogeneous, unfissured rock, 

 and a second type, a pure explosion form, should be recognized in a 

 full classification of vents, 



A diatreme of either kind may be enlarged by the continued passage 

 of the blowpiping gases, by the mechanical erosion of the walls by out- 

 flowing lava, or by the piecemeal stoping of the walls by the lava 

 column. 



Plutonic Cu]X)las. — Lastly, a complete genetic scheme should recog- 

 nize a process of vent-opening which is neither explosion, nor the enlarge- 

 ment of through-going fissures. Most of the greater abyssal injections 

 are strictly intrusive and do not occasion the outflow of magma at the 

 earth s surface. Those of batholithic size show, in the field, clear evi- 

 dence of having actually worked their way up the last few hundred 

 meters or last few kilometers, before the respective magmas have solid- 

 ified. The process is one of absorjjtion of the roof-rock, and is ckvirly 

 distinct from that of mere injection, either abyssal or satellitii*. Both 

 stoping and marginal assimilation are most rapid at the hottest parts 

 of the contact Itetween magma and country-rock. At the roof, the 

 juvenile gases tend to accumulate in any cupola-like irregularities in 

 the roof. These gases rise from the interior of the magma, i)erhaps 

 from great depth. Because of the pressure reigning even at the roof 



