Conditioned on the one side by the known fault of the San Joaquin Valley and on the other by the volcanic activity of the Pacific basin, the greater peninsula of San Francisco in particular has always been subject, so far as the memory of white settlers can go, to frequent shocks of earthquake. In the last score or more of years seismographic observatories have been maintained at several points about San Francisco bay, and the records have been sufficiently studied to afford data for comprehension of the varied earth waves which have made themselves felt either to the perception of the citizens of the Golden Gate or to the sensitive instruments. Such observations have been conducted by Prof. George Davidson, for many years in charge of the Coast and Geodetic Survey upon the Pacific Coast; by Prof. Charles Burckhalter, of the Chabot Observatory, in Oakland, and by the staff of the Lick Observatory on Mount Hamilton. Careful inspection of these records shows that two systems of earthquake disturbances act upon San Francisco. Those of the lighter series show a wave movement beginning in one of the easterly quadrants and more commonly in the southeastern. This series of light shocks is attributed to the slip along the line of the San Joaquin fault. While they may occur at any season of the year, they are more frequently observed when the San Joaquin river is running bank high under the influence of the melting snows in the foothills of the Sierra. That such a condition has recently existed is made clear by the report within less than a month of floods in the interior valleys of the State. Assuming, as the geologists do, that the fault in the valley lies near the roots of the Monte Diablo range, on the western edge of the alluvial plain, it will be seen that the physical factors involving the slip are very simple. There is a wide, flat plain bounded on the west by a line of weakness in the rock supports. When this plain is carrying an abnormal weight of water the tendency is to break downward at the line of the fault. This tendency will produce a jar in the mountain mass which will be rapidly communicated to its farthest extremity. The earthquakes which have their origin in the disturbances to which the oceanic basin is subject always approach San Francisco from the direction of the southwest quadrant. These have been uniformly more violent than those whose origin is attributed to the San Joaquin fault. While the records of San Francisco earthquakes up to the present have exhibited a mild type, the damage to property having hitherto been slight, it would appear from the extent and violence of the present temblor that both causes had for once united.