Of 948 earthquake shocks that have been recorded in California previous to 1887, 417 were most active in San Francisco. The seismographs which record the merest tremors and determine the place of the shock show that 344 have occurred since 1888. Half of the sum total have occurred in the vicinity of the gate city and for this reason it is believed that the severe shock of April 18 was the final fall of a crust of the earth which has been gradually slipping for centuries, causing from time to time the slight shocks. The seismic physics of San Francisco and its immediate neighborhood have engaged the careful study of physical geographers. The commonly accepted opinion is one which was formulated by Prof. John Le Conte, professor of geology in the University of California, and one of the world's geological authorities. His explanation is based upon the mountain contours of the coast of California from the Santa Barbara channel northward to the Golden Gate. In this region are represented two peninsulas, one visible, the other to be discovered through examination of the altitudes upon the map corresponding to existing geological features. This second and greater peninsula comprises the Monte Diablo and Coast ranges, separated from the Sierra elevation by the alluvial soil of the low-lying valley of the San Joaquin. This valley is contoured by the level of 100 feet and lower for a considerable portion of its length, and practically all of it lies below the level of 500 feet. The partition thereby accomplished between the Sierra mountain mass and the coastal mountains is sufficiently pronounced to indicate what was at no remote period an extensive peninsula. This valley of the San Joaquin lies above the line of a geological fault, at a depth which can only be estimated as somewhere about a mile. The artesian well borings which have been abundantly prosecuted in the counties of Merced, Fresno, Kings and Kern afford evidence looking toward such a determination of bedrock depth. On the ocean side the continental shelf is extremely narrow. The great peninsula presents a most precipitous aspect toward the ocean basin. It is interrupted at intervals by deep submarine gorges extending close to the shore. The oceanic basin of the Pacific is throughout a region of volcanic upheaval and seismic disturbance.