REVIEWS 289 



observations made, unmodified by special views of origin; and such a report 

 must always remain of the greatest value, however much theories should 

 change. A few pages only are devoted to discussion of results, and theories 

 of cause are either briefly set forth or implied rather than explicitiy stated. 

 By many this will be regarded as unfortunate, since they will wish to know 

 the lines of evidence by which the conclusions were reached. It appears 

 with sufficient clearness that in the view of the chairman of the commission, 

 the disturbance of April 18, 1906, was due entirely to a diiJerential mass 

 movement of the ground upon a single surface (or narrow zone) of fracture — 

 the so-called St. Andreas Rift; and that all earlier historic earthquakes 

 within the same province, with the exception of that of 1868, were likewise 

 caused by movements upon this same rift. The earthquake of 1868 is 

 ascribed to a similar adjustment upon the degraded fault along the north- 

 east margin of the Santa Clara Valley and San Francisco Bay. Other 

 possible displacements, within the same province at the same times, have 

 in the report been excluded from consideration, perhaps because none 

 were revealed through surface breaks. 



It has further been assumed that the local energy of the disturbance 

 w^as determined solely by distance from the rift, the apparent surface 

 intensity, when not in harmony with this law, being accounted for solely 

 by differences of elasticity within the sub-surface materials. This is 

 certainly of very great interest, provided it is true ; but the maps and printed 

 observations do not make it by any means conclusive. Map No. 23 of the 

 atlas (distribution of apparent intensity) does not make it appear even 

 probable. Dr. G. K. Gilbert, a member of the commission, some four 

 months after the earthquake, published a short paper in which he advanced 

 the same theory, though he significantly added: 



But after making due allowance for differences in natural foundation and for 

 differences in the resisting power of buildings, there remain various anomalies 

 for which satisfactory explanation has not as yet been found. The natural founda- 

 tion of Oakland is similar to that of San Jose, and its distance from the earth- 

 quake origin is about the same, but the injury to its buildings was decidedly less; 

 and Santa Rosa, standing on ground apparently firmer than that at Oakland or 

 San Jose and having a somewhat greater distance from the fault, was nevertheless 

 shaken with extreme violence.^ 



It is too early to discuss these anomalies. With the data now in hand it 

 seems to be true that there are outlying tracts of high intensity surrounded by 

 areas of relatively low intensity; and these features, if they shall be fully estab- 

 lished, will doubtless affect in some important way the general theory of the 

 earthquake. 



I The italics are mine. — W. H. H. 



