Page:Popular Science Monthly Volume 69.djvu/296

292 preceded the recent one. In it the aggregate displacement horizontally has been very great, and the aggregate vertical displacement as shown by the rock strata on either side of it exceeds half a mile.

It is the purpose of this article to trace the earthquake rift of April 18, 1906, across the map of California. The accompanying photograph of a relief map by Dr. Noah Fields Drake will show the topography of the state. In California there are multitudes of valleys of various kinds. Those formed by water and ice surface erosion are variously curved and ramified. Such are the mountain canons of the west flanks of the Sierras. Those valleys formed or marked by earthquake cracks have almost invariably straight axes. These extend in general toward the north-northwest, more or less distinctly parallel with each other, and often intersected by cross-faults.

Examples of faulted valleys are the great valley of the Sacramento and San Joaquin, the Santa Clara Valley, San Francisco Bay, with the Valley of Santa Rosa, Eel River Valley, the Santa Catalina Channel, Owens River, the San Jacinto Valley and many others. A cross-fault extends from Monterey Bay up the valley of the Pájaro River. In some of these faults earthquakes have taken place in historic times, in others no break has been noted save that recorded in the rocks. Dr. Branner has compared a fault to a break in a bone. It represents a weak place which will give in a time of strain. On the other hand, if not freshly broken, it will tend with time to heal. A broken bone