CAUSES OF VULCANISM 245 



that they bore their way through the zone of fracture. In the 

 driiM-r and warmer zone below, the alternatives seem to be (i) 

 nurhanicul penetration without fracture, or (2) melting or fluxing. 

 As rocks "flow" in this zone by differential pressure without rup- 

 ture, an included liquid mass may perhaps be forced to flow through 

 the zone by differential pressure. Lava probably fuses or fluxes its 

 way, under pressure, through the rock below the zone of fracture. 

 In this it may be supposed to be assisted by its gases, by the selective 

 nature of its fluxing, by its exceedingly high temperature if it comes 

 from very great depths, and by the stress-differences which attend 

 tidal strains in the deep interior. In ascending, the lava would be 

 invading regions of lesser pressure and lower melting-point. It 

 would therefore have heat in excess of the local melting temperature, 

 until it reached the cool rock. From that point on, the rising lava 

 must constantly lose temperature by contact with cool rocks. If 

 its excess of temperature is insufficient to enable it to reach the zone 

 of fracture, the ascending column is arrested and becomes plutonic 

 rock. If it suffices to reach the zone of fracture, advantage may be 

 taken thereafter of fissures, and the problem of further ascent 

 probably becomes chiefly one of hydrostatic 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 of the 

 rock of the lower horizons. The ascending column may reach 

 hydrostatic equilibrium before it reaches the surface, and then 

 form intrusions of various sorts, or it may find equilibrium only by 

 coming to the surface. 



References. C. E. Button, Hawaiian Volcanoes, Fourth Ann. Kept., U. S. 

 Geol. Surv., 1883. Judd, Volcanoes, 1881; J. D. Dana, Characteristics of Vol- 

 canoes, 1890. A. Geikie, Ancient Volcanoes of Great Britain, 1897. I. C. Russell, 

 Volcanoes of North America, 1897. T. G. Bonney, Volcanoes, Their Structure 

 and Significance, 1899. A. Heilprin, Mont Pelee and the Tragedy of Martinique, 

 1903. Accounts of same volcanoes in the Nat'l. Geog. Mag., Vol. XIII, 1902 

 (Russell, Hill, Hovey, Dillcr, and Hildebrand). 



Map work. See Plates CLV to CLXIV, of Professional Paper 60, U. S. Geol. 

 Surv. and Exercise XVI, in Interpretation of Topographic Maps. 



