Page:Popular Science Monthly Volume 69.djvu/73

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69
THE SAN FRANCISCO EARTHQUAKE
THE GEOLOGICAL PRELUDE TO THE SAN FRANCISCO EARTHQUAKE
BY GEO. H. ASHLEY
WASHINGTON, D. C.

SINCE 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 activities 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 résumé 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 activity—the movements of subsidence, the vast volume of sedimentation, the intrusion of great sheets of igneous rocks, and the final folding, 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 immediate neighborhood of San Francisco consisted of a subsidence beginning 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