THE SAN FRANCISCO EARTHQUAKE 69 



THE GEOLOGICAL PRELUDE TO THE SAX FRANCISCO 



EARTHQUAKE 1 



BY GEO. H. ASHLEY 

 WASHINGTON, D. C. 



CJINCE the San Francisco earthquake, the reading and scientific 

 ^ public has become acquainted with the fact, if not already known, 

 that the recent disaster was the result, not of volcanic activity, but of 

 the activity of the ordinary mountain-making forces. In a large 

 measure they have become acquainted with the further facts that 

 mountain-making forces have long been, and still are, active in the 

 immediate region about San Francisco; that as a result of these ac- 

 tivities the rocks of the regions are folded and faulted; that the 

 faulting is of major importance ; that the recent disturbance is ascribed 

 by the geologists to movements of adjustment along one or more of 

 these fault planes; that investigation after the earthquake along some 

 of these lines gave abundant evidence of differential movement visibly 

 affecting the surface. 



These facts, now widely known, start questions along several lines 

 of inquiry. One of these lines, having an important bearing on the 

 probabilities of future trouble, involves the geologic evidence as to the 

 recency of the observed earth movements; the relative value of this 

 last displacement as compared with past displacements, both remote 

 and near; the character and amount of geologically recent movements; 

 in short, as given in this paper, a resume of the recent geologic history 

 of the San Francisco peninsula and the observed evidence upon which 

 the statement of that history is based. 



As our interest increases with the recency of the events the earlier 

 history will be passed over rapidly and increasing attention given to 

 the later events. 



California in Mesozoic time was the theater of profound geologic 

 activitv — the movements of subsidence, the vast volume of sedimenta- 

 tion, the intrusion of great sheets of igneous rocks, and the final fold- 

 ing, crushing and faulting were possibly not exceeded anywhere in 

 the world during that period. In Tertiary time the same notable 

 activity continued. The last expression of that activity in the im- 

 mediate neighborhood of San Francisco consisted of a subsidence be- 

 ginning apparently just at or before the end of Miocene time and 

 continuing probably a little over into the Quaternary. Coincident with 

 this subsidence was sedimentation that locally resulted in the laying 

 down of over 4,700 feet of sediments. A remnant of these deposits, 

 known as the Merced series, stretches from the city limits of San 



