390 



GEOLOGY 



and thence, taking advantage of fissures and fractures, reach the 

 surface (Fig. 301). It is conceived that such liquefaction and 

 extrusion would carry the excess of temperature received by the 



middle zone from the deeper 

 interior, out toward the sur- 

 face, or even to it. The out- 

 ward movement of the lava 

 would tend to regulate the 

 temperature of the middle 

 zone, forestalling general lique- 

 faction, and keeping the zone 

 as a whole solid. The inde- 

 pendence of volcanoes is as- 

 signed to the independence of 

 the liquid threads that work 

 their way to the surface. Noth- 

 ing like a reservoir or molten 

 lake enters into the conception. 

 The prolonged action of vol- 

 canoes is attributed to the slow 

 feeding of the liquid threads 



zone; a'-f, zone of continuous rock from the middle zone, which 

 below surface melting temperature; 

 ff-c, interior portion whose temper- 

 atures rise from the surface melting- 

 point at /-/ to a maximum at C; 

 V, V, threads or tongues of molten 

 rock rising from the interior to vari- 

 ous levels, many of these lodging 

 within the fragmental zone as 

 tongues, batholiths, etc.; PPP, ex- 

 plosion pits formed by volcanic gases 

 derived from tongues of lava below. 



Fig. 301. Ideal section of a portion 

 of the early earth, illustrating its 



is liquefied in spots only. The 

 frequent pauses in volcanic 

 action are assigned to tempo- 

 rary deficiencies of supply, and 

 the renewals to the gathering 

 of new supplies after a suffi- 

 cient period of accumulation. 



The distribution of volcanoes 

 in essentially all latitudes and longitudes is assigned to the 

 general nature of the cause. The special surface distribution 

 is assumed to be influenced, though not altogether controlled, by 

 the favorable or unfavorable conditions for escape presented at 

 different places. The persistence of volcanic action in time is 

 attributed to the magnitude of the interior source, to its deep- 

 seated position, and to the slowness of conduction of heat from the 



