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.

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 mai