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POPULAR SCIENCE MONTHLY 



preceded the recent one. In it the aggregate displacement horizon- 

 tally 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 photo- 

 graph of a relief map by Dr. N~oah Fields Drake will show the topog- 

 raphy 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 earth- 

 quake 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 Eosa, Eel Eiver Valley, the Santa Catalina Channel, 

 Owens Eiver, the San Jacinto Valley and many others. A cross-fault 

 extends from Monterey Bay up the valley of the Pajaro Eiver. 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 



Alder Creek Bridge, Mendocino County. The earthquake rift is near the 



middle of the picture. 



