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SCIENCE 



[N. S. Vol. XXIX. No. 734 



to several miles, are numerous ridges and 

 troughs, long or short, level or inclined, 

 and approximately parallel to the trend 

 of the belt. Each of these represents a 

 dislocated tectonic block, and the disloca- 

 tion is of so recent date that the disturbed 

 drainage has made little progress toward 

 the restoration of normal conditions. 

 Lakelets are numerous, and streams 

 wander irregularly. "Without delaying to 

 attempt a fuller and more adequate 

 description, which may be found in the 

 report of the California Commission,^ I 

 content myself with an assurance to physi- 

 ographers that the topographic expression 

 of the rift belt is distinctive, so that it can 

 readily be recognized in other localities by 

 those who have made its personal ac- 

 quaintance in the field. Other belts of the 

 same character have already been found 

 in California^ and their discovery else- 

 where may confidently be expected. 



Rift topography appears to be the sur- 

 face expression of a species of repetitive 

 horizontal faulting, just as the fault scarp 

 is the surface expression of vertical f ault- 



' California earthquake of April 18, 1906 ; re- 

 port of the State Earthquake Investigation Com- 

 mission; published by the Carnegie Institution, 

 Washington, 1908, pp. 25-52. 



° The only rift belt beside the San Andreas which 

 has yet been traced for any distance is one which 

 follows in a general way the western base of the 

 Berkeley hills. In the vicinity of Oakland its 

 position is indicated by a trough among the lower 

 hills two or three miles back from the piedmont 

 plain. At Haywards it coincides with the western 

 base of the hills, and at Irvington, with the 

 western base of a projecting spur. In Berkeley 

 also its line follows the base of the hills, but a 

 little northward it climbs to the summit of the 

 first ridge. The principal fault occasioning the 

 earthquake of 1868 was in this rift belt, running 

 from Haywards southward, and it is probable that 

 some of the earlier recorded earthquakes were 

 associated with the same belt. The fault of 1868 

 is described, and the rift belt is mentioned, in the 

 " Report of the California Earthquake Commis- 

 sion," Vol. I., Part II., pp. 434-5 and 447. 



ing, and the two types, which with present 

 knowledge are apparently distinct, will 

 doubtless eventually be found to inter- 

 grade. The features of the San Andreas 

 rift — the one associated with the San 

 Francisco earthquake— were neither cre- 

 ated nor greatly modified at the time of 

 that shock, but such modifications as were 

 made were of such character as to accentu- 

 ate and perpetuate the peculiarities of the 

 belt. The belt itself would be the natural 

 result of a long series of such events, suc- 

 ceeding one another with such rapidity 2& 

 to dominate minor aqueous agencies in the 

 modeling of the surface. These considera- 

 tions, together with the fact that earth- 

 quakes are known to have repeatedly 

 originated in the rift belts of California, 

 serve to establish the rift topography as a 

 criterion for the recognition of malloseis- 

 mic districts. 



Geologic Formationj—FaMlt scarps and 

 rift belts serve to indicate some of the 

 foci of past and future earthquakes. 

 Other foci lie wholly within the earth's 

 crust. Whether the rupture occurs above 

 or below, its jar is propagated through the 

 crust in all directions and affects a large 

 area of the surface. Within this area the 

 intensity of the shock varies primarily 

 with distance from the origin, but it varies 

 also with the character of the geologic 

 formation at the point of emergence. The 

 variation with formation has less range 

 than the variation with distance, but is not 

 less important to the resident and the so- 

 journer, the architect and the engineer — 

 that is to say, it is equally important in 

 forecasting areas of dangerous energy. 

 The portion of San Francisco most in- 

 tensely racked by the shock of 1906 stood 

 farther from the fault line than the portion 

 least affected, but it stood on less coherent 

 soil. Wood has carefully mapped the dis- 

 tribution of intensity in the San Francisco 

 peninsula, as evidenced by the injury to 



