=Fold _versus_ fault topography.=—The more resistant or “stronger” rock beds, as regards attacks of the atmosphere, in the course of time come to stand in relief, separated by depressions which overlie the “weaker” formations. Simple open folds which are not plunging exercise an influence upon topography by producing generally long and straight ridges. More complex flexures, since they generally plunge, make themselves apparent by features which in the map are represented by curves. Fracture structures, and especially block displacements, are differentiated from these curving features by the dominance of straight or nearly rectilinear lines upon the map. The effect of erosion is to reduce the asperity of features and to mold them with flowing curves. The fracture structures are for this reason much more likely to be overlooked, and if they are not to elude the observer, they must be sought out with care. Fold and fracture structures may both be revealed upon the same map. READING REFERENCES TO CHAPTER VI Joint systems:— JOHN PHILLIPS. Observations made in the Neighborhood of Ferrybridge in the Years 1826-1828, Phil. Mag., 2d ser., vol. 4, 1828, pp. 401-409; Illustrations of the geology of Yorkshire, Pt. II, The Limestone District. London, 1836, pp. 90-98. SAMUEL HAUGHTON. On the Physical Structure of the Old Red Sandstone of the County of Waterford, considered with reference to cleavage, joint surfaces, and faults, Trans. Roy. Soc. London, vol. 148, 1858, pp. 333-348. W. C. BRÖGGER. Spaltenverwerfungen in der Gegend Langesund-Skien, Nyt Magazin for Naturvidernskaberne, vol. 28, 1884, pp. 253-419. WM. H. HOBBS. The Newark System of the Pomperaug Valley, Connecticut, 21st Ann. Rept. U. S. Geol. Surv., Pt. III, 1901, pp. 85-143. Geological map:— WM. H. HOBBS. The Interpretation of Geological Maps, School Science and Mathematics, vol. 9, 1909, pp. 644-653. CHAPTER VII THE INTERRUPTED CHARACTER OF EARTH MOVEMENTS: EARTHQUAKES AND SEAQUAKES =Nature of earthquake shocks.=—Man’s belief in the stability of Mother Earth—the _terra firma_—is so inbred in his nature that even a light shock of earthquake brings a rude awakening. The terror which it inspires is no doubt largely to be explained by this disillusionment from the most fundamental of his beliefs. Were he better advised, the long periods of quiet which separate earthquakes, and not the lighter shocks which follow all grander disturbances, would occasion him concern. [Illustration: FIG. 49.—View of a portion of the ruins of Messina after the earthquake of December 28, 1908.] Earthquakes are the sensible manifestations of changes in level or of lateral adjustments of portions of the continents, and the seismic disturbances upon the sea—seaquakes and seismic sea waves—relate to similar changes upon the floor of the ocean.