THE COMPOSITION OF THE EARTH 59 



tive age of the igneous and sedimentary rock (Fig. 42) (a) if the former 

 is extrusive, (6) intrusive ? (2) What hypotheses may be advanced 

 to account for the isolated areas of sedimentary rock within the 

 igneous rock area ? How could these theories be tested in the field ? 

 (3) How could one determine in the field whether the igneous 

 rock is intrusive or extrusive ? 



REFERENCES 

 MINERALS AND ROCKS 



DANA : Minerals and How to Study Them. 2d ed. (New York, 



1902.) 



GEIKIE, J. : Structural and Field Geology. (New York, 1905.) 

 KEMP : A Handbook of Rocks. 2d ed. (New York, 1900.) 

 PIRSSON : Rocks and Rock Minerals. (New York, 1908.) 



VOLCANOES, ETC. 



BONNET : Volcanoes; Their Structure and Significance. (New York, 



1899.) 

 DILLER : ML Shasta, a Typical Volcano, in Physiography of the 



United States, pp. 237-268. (New York, 1895.) 



The Geology and Petrography oj Crater Lake National Park; 

 Prof. Paper No. 3, U.S. Geol. Surv. 



BUTTON : Hawaiian Volcanoes, in 4th Ann. Rept., U.S. Geol. 

 Surv., pp. 81-219. 



Mount Taylor and the Zuni Plateau, in 6th Ann. Rept., U.S. 

 Geol. Surv., pp. 105-198. 



GILBERT : Geology of the Henry Mountains, pp. 18-60 ; U.S. Geog. 

 and Geol. Surv., Rocky Mt. Region. (Washington, 1877.) 



HEILPRIN : Mont Pelee and the Tragedy of Martinique. (Phila- 

 delphia, 1903.) 



HILL : Report on the Volcanic Disturbance in the West Indies, in 

 Nat. Geog. Mag., Vol. XIII, pp. 223-267. 



HOVEY : The Eruption of La Soufriere, St. Vincent, in May, 1902, 

 in Nat. Geog. Mag., Vol. XIII, pp. ill 159. 



JAGGAR : The Eruption of Mount Vesuvius, April 7-8, 1906, in 

 Nat. Geog. Mag., Vol. XVII, pp. 318-324. 



JUDD : Volcanoes. (New York, 1893.) 



RUSSELL : Volcanoes of North America. (New York, 1897.) 



The Recent Volcanic Eruptions in the West Indies, in Nat. 

 Geog. Mag., Vol. XIII, pp. 267-285. 



Volcanic Eruptions on Martinique and St. Vincent, in Nat. 

 Geog. Mag., Vol. XIII, pp. 415-436. 



