392 GEOLOGY 



relatively cool, and their capacity for melting is low, or perhaps 

 even gone. 



In the denser and warmer zone below, the alternatives seem to 

 be (1) melting or fluxing, or (2) mechanical penetration without 

 fracture. As rocks "flow" in this zone by differential pressure 

 without rupture, an included liquid mass may be forced to flow 

 through the zone by sufficient differential pressure. If local differ- 

 ential pressures at the surface are neglected as probably incompetent, 



Fig. 302. Mount Baker, Wash. An old volcanic cone. (Copyright, 1906, 

 by Riser Photo. Co., Portland, Ore.) 



there only remain the stress-differences of the interior, and the 

 differences of hydrostatic pressure between the lava-column and 

 the surrounding solid columns. The latter would not be great 

 until a column of liquid of much depth was formed, and the former 

 would probably not be concentrated on the liquid in such a way 

 as to force it bodily through the solid rock. Probably fusing or 

 fluxing its way with the aid of stress-differences is the chief cause 

 of the rise of lava below the zone of fracture. In this it may be 

 supposed to be assisted by its gases, by its selective fusible and 

 fluxing nature, by its very high temperature if it comes from very 

 great depths, and by the stress-differences which attend tidal 

 strains in the deep interior. In ascending from lower to higher 



