632 GEOLOGY. 



and mingling with the lava, or else sinking to lower positions in the 

 column. This process he designates storing} 



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 differential pressures at the 

 surface be neglected as probably incompetent, there only remain the 

 stress-differences of the interior and the differences of hydrostatic pres- 

 sure 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 resource of the lava in the initial stages. In this it may be 

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

 nature, b}^ its very high temperature if it comes from very great depths, 

 as held in the seventh hypothesis, and by the stress-differences which 

 prevail in the deep interior, as shoAvn in the last chapter. In ascending 

 from lower to higher horizons, the lava would be constantly invading 

 regions of lower melting-point because of less pressure. It would thus 

 always have an excess of heat above the local melting temperature until 

 it invaded the external, cool zone, where the regional temperature is 

 below the melting-point of surface pressure. From that point on it 

 must constantly lose portions of its excess of temperature by contact 

 with cooler rocks, and probabl}^ in the process of fluxing its way in the 

 compact zone. If this excess is insufficient to enable it to reach the 

 zone of fracture, the ascending column is arrested and becomes merely 

 a plutonic pipe or mass. If it suffices to reach the zone of fracture, 

 advantage may be taken thereafter of fissures and of rupturing, and 

 the problem of further ascent probably becomes chiefly one of hydro- 

 static pressure, in which the ascent of the lava-column is favored by its 

 high temperature and its included gases. The hydrostatic contest is 

 here between the lava- column, measured to its extreme base, and the 

 adjacent rock-columns, measured to the same extreme depth. The result 

 is, therefore, not necessarily dependent on the flowage of the outer rocks, 

 but may be essentially or wholly dependent on the deep-seated flowage 

 1 Mechanics of Igneous Intrusion, Am. Jour. Sci., Apr., p. 269, and Aug., p. 107, 1903. 



