VULCANISM 393 



horizons, the lava would be constantly invading regions of lower 

 melting-point, because of lesser pressure, and thus always have an 

 excess of heat above the local melting temperature until it invaded 

 the external, cool zone. From that point on, the rising lava must 

 constantly lose portions of its excess of temperature by contact 

 with cooler 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 may then form 

 underground intrusions of various sorts without superficial eruption, 

 or it may only find equilibrium by coming to the surface and pour- 

 ing out a portion of its substance and discharging its gases. 



References on vulcanism. G. P. Scrope, Volcanoes, London, 1872. C. E. 

 Button, Geology of the High Plateaus of Utah, U. S. Geog. and Geol. Surv., 

 1880. The Hawaiian Volcanoes, Fourth Ann. Kept., U.S. Geol. Surv., 1883. 

 Judd, Volcanoes, 1881; The Eruption of Krakatoa (Com. of the Roy. Soc.), 

 1888. J. D. Dana, Characteristics of Volcanoes, 1890. Milne and Burton, 

 The Volcanoes of Japan, 1892. J. P. Iddings, The Origin of Igneous Rocks, 

 Bull. Phil. Soc., Washington, Vol. XII, 1892. A. C. Lane, Geologic Activity 

 of the Earth's Originally Absorbed Gases, Bull. Geol.' Soc. Am., Vol. V, 1894. 

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

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

 and Significance, New York (and London), 1899. A. Heilprin, Mont Pelee 

 and the Tragedy of Martinique, Philadelphia (and London), 1903. Further 

 accounts of the same volcanoes are found in the Nat'l Geog. Mag., Vol. XIII, 

 1902 (Russell, Hill, Hovey, Diller, and Hildebrand). 



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

 Geological Survey, illustrate various topographic effects of vulcanism. The 

 Structure Section Sheets of the folios of the Survey, and the maps of various 

 Survey Reports show the many and diverse relations of igneous rocks. 



