W. V. KNEBEL. Höhlenkunde, etc., Braunschweig, 1906, pp. 175-177 (the blue grotto of Capri). Oscillation of movement:— C. LYELL. Principles of Geology, vol. 2, pp. 164-176 (Temple of Jupiter Serapis). E. RAY LANKESTER. Extinct Animals, New York, 1905, pp. 31-42. H. W. FAIRBANKS. Oscillations of the Coast of California during the Pliocene and Pleistocene, Am. Geol., vol. 20, 1897, pp. 213-245. G. H. STONE. Mon. 34, U. S. Geol. Surv., 1899, pp. 56-58, pl. 2. BAILEY WILLIS. Ames Knob, North Haven, Maine. Bull. Geol. Soc. Am., vol. 14, 1903, pp. 201-206, pls. 17-18. Simultaneous contrary movements on a coast:— A. C. LAWSON. The Post-Pliocene Diastrophism of the Coast of Southern California, Bull. Univ. Calif. Dept. Geol., vol. 1, 1893, pp. 115-160, pls. 8-9. W. S. TANGIER-SMITH. A Geological Sketch of San Clemente Island, 18th Ann. Rept. U. S. Geol. Surv., Pt. ii, 1898, pp. 459-496, pls. 84-96. R. S. TARR and L. MARTIN. Recent Changes of Level in the Yakutat Bay Region, Alaska, Bull. Geol. Soc. Am., vol. 17, 1906, pp. 29-64, pls. 12-23. CHAPTER XX THE GLACIERS OF MOUNTAIN AND CONTINENT =Conditions essential to glaciation.=—Wherever for a sufficiently protracted period the annual snowfall of a district is in excess of the snow that is melted, a residue must remain from each season to be added to that of succeeding ones. Eventually so much snow will have accumulated that under its own weight and in obedience to its peculiar properties, a movement will begin within the mass tending to spread it and so to reduce the slope of its upper surface (Frontispiece plate). The conditions favorable to glaciation are, therefore, heavy precipitation and low annual temperature. If the precipitation is scanty, the small snowfall is soon melted; and if the temperature be too high, the moisture is precipitated not in the form of snow but as rain. It is important here to keep in mind that snow is a poor heat conductor and itself protects its deeper layers from melting.