THE EXTRUSIVE PROCESSES. 591 



participates in the outward movement. In so far as molecular action 

 permits a readjustment of material, there is a tendency, even in the 

 solid state, for the lighter material to move upwards and the heavier 

 downwards, and for the more stressed portions to move toward points 

 of less stress; but this takes place with extreme slowness. In so far 

 as the materials of the interior diffuse themselves through each other, 

 the same laws hold good, but they are modified by the special principles 

 that control diffusion. The outward diffusion of interior gases may be 

 a factor of appreciable importance, but this cannot be affirmed at 

 present. 



Phases of vulcanism. — The forcing of fluid rock outward assumes two 

 general phases, which, however, merge into each other; and these main 

 phases take on various sub-phases. The first phase embraces those 

 outward movements of fluid rock which do not reach the surface. The 

 lavas, after ascending to the vicinity of the surface, intrude themselves 

 into the outer formations of the earth and congeal underground (plu- 

 tonic). The second phase embraces those outward movements in wliich 

 the fluid rock reaches the surface and gives rise to eruptive phenomena 

 (volcanic). The first is intrusive, the second extrusive; the first consti- 

 tutes irruptions, the second eruptions.'^ The f\mdamental nature of 

 the two is the same, but the extrusions usually take on special phases 

 because of the relief of pressure at the surface of the earth, and be- 

 cause of the action of surface-waters in contact AAith the heated lavas. 

 Just where the lavas come from, and how they find their way through 

 the deep-lying compact zone below the zone of fracture, may better 

 be considered later. When they reach the zone of fracture, they usu- 

 ally either take advantage of fissures already formed, or force passage- 

 ways for themselves by fracture. There is little e\idence that they 

 bore their way through the rocks by melting, though they appear to 

 round out their channels in some way into pipes, ducts, and other 

 tubular forms when they flow through them for long periods of time. 



1. Intrusions. 



Fluid rock forced into fissures and solidified there forms dikes; 

 forced into chimney-like passages, it forms pipes or plugs; insinuated 



^ The terms are here used in their narrow technical sense. Extrusion is also used in 

 a broad generic sense to indicate the whole process of outward movement. 



