In its youthful stage, the cinder cone usually presents a broad summit sag and relatively short side slopes, whereas the cone of later stages is apt to present long sweeping and upwardly concave curves with both the gradient and the radius of curvature increasing rapidly toward the summit. In contrast, too, with the earlier stage, the crest is relatively small. A marked reduction in the high symmetry of such profiles is noted wherever a breaching by lava outflow has occurred (Fig. 154). With the composite cone, complexity and corresponding lack of symmetry is introduced, especially in the partially ruined caldera, and by the more or less accidental distribution of parasitic cones, as well as by migrations of the central cone. Peculiarly similar acuminated profiles result from spatter-cone formation, from the formation of a superchimney spine, and by the uncovering of the chimney through denudational processes—the volcanic neck. [Illustration: FIG. 154.—Character profiles connected with volcanoes.] Another important feature resulting from denudation is the Mesa or table mountain with its protecting basalt cap above softer rocks. Its profile most resembles that of table mountains due to differential erosion of alternately strong and weak horizontally bedded rocks, such as compose the upper portion of the section in the Grand Cañon of the Colorado. Here, however, in place of a single unusually strong top layer there are found several strong layers in alternation with weaker ones so as to produce additional steps in the profile. READING REFERENCES TO CHAPTERS IX AND X General works:— PAULETT SCROPE. The Geology of the Extinct Volcanoes of Central France. John Murray, London, 1858, pp. 258. (An epoch-making work of early date which, like the following reference, may be studied to advantage to-day.) SIR CHARLES LYELL. Principles of Geology, vol. 1, Chapters xxiii-xxv. MELCHIOR NEUMAYR. Erdgeschichte, vol. 1, Allgemeine Geologie, revised edition by v. Uhlig, 1897, pp. 133-277 (a storehouse of valuable information clearly presented). J. D. DANA. Characteristics of Volcanoes, with Contributions of Facts and Principles from the Hawaiian Islands. Dodd, Mead, and Company, New York, 1890, pp. 397. TEMPEST ANDERSON. Volcanic Studies in Many Lands, being reproductions of photographs by the author with explanatory notes. John Murray, London, 1903, pp. 200, pls. 105. T. G. BONNEY. Volcanoes, their Structure and Significance. John Murray, London, 1899, pp. 331. I. C. RUSSELL. Volcanoes of North America. Macmillan, New York, 1897, pp. 346. ELISÉE RÉCLUS. Les volcans de la terre, Belgian Society of Astronomy, Meteorology, and Physics of the Globe, 1906-1910 (a valuable descriptive geographical and bibliographical work of reference). G. MERCALLI. I vulcani attivi della terre. Hoepli, Milan, 1907, pp. 421. (A most valuable work, beautifully illustrated, but in the Italian language.) Arrangement of volcanic vents:—