Earthquakes sometimes cause sudden displacements, horizontal or vertical, of sufficient amount to affect the information shown on the charts. A careful investigation of the effects of the earthquake in Yakutat Bay, Alaska, in September, 1899, showed that the shore was raised in some parts with a maximum uplift of 47 feet and depressed in other parts, and that at least two reefs and four islets were raised in the water area where none appeared before. Undoubtedly there were changes in the water depths, but definite information is lacking because there had been no previous hydrographic survey. The San Francisco earthquake of 1906 caused little vertical displacement, but there were horizontal changes of relative position as much as 16 feet; so far as known this earthquake did not affect the practical accuracy of the charts. Related to earthquake phenomena are the gradual coast movements of elevation or subsidence which are taking place but at so slow a rate as not to sensibly affect the charts in ordinary intervals of time. Another agency at work is the coral polyp on the coral reefs; although the rate of growth appears to be very slow, the resulting reefs and keys are an important feature in tropical seas. Practically all of the land features shown on charts are likewise subject to changes, the more rapid of which are mainly due to the works of man. The changes of channels and of commercial needs cause many alterations to be made from time to time in the lights and buoys which are shown on the charts. #Methods of correction.# The problem of keeping a chart sufficiently up to date is one of much practical importance and one which must be taken into account in planning what should be shown on the chart in the first place so as to bring it within the range of practicable revision. Certain features are corrected at once on the charts as soon as the information is received, such as dangers reported, and changes in lights and buoys. Where harbor works are in progress the periodic surveys made in this country by the Corps of Engineers furnish data which are applied promptly to the charts. Reported dangers in channels and bars are investigated by special surveys and the information is put on the charts. Examinations are made from time to time for the revision of the features along the coast line. Complete resurveys have been made, at long intervals, of some important portions of the coast where there has been evidence of change, and these, when they become available, are applied to the charts. All parts of the coast where the exposed portions are not of very permanent material will require resurveys at intervals, depending on their importance and the rate of change.