Januaby 22, 1909] 



SCIENCE 



123 



Other bases for forecast are connected 

 with our conceptions as to the origin of 

 earthquakes. The theory of earthquakes 

 now generally accepted ascribes them to 

 the sudden breaking or slipping of rocks 

 previously in a condition of shearing 

 strain. Exception should probably be 

 made of some of the shocks accompanying 

 volcanic eruptions, but volcanic shocks 

 constitute a class by themselves to which 

 it is not important to extend the present 

 discussion. In non-volcanic, or ordinary, 

 examples it is believed that the strains 

 arise in connection with those tectonic or 

 diastrophic changes which are exhibited 

 superficially in the deformation of the 

 surface, and that their accumulation is 

 gradual. Fracture occurs when and 

 where the internal stress exceeds' the 

 strength of the rock, and a fault results. 

 Slipping takes place when the stress along 

 the plane of a preexistent fault exceeds 

 the force of adhesion. In either case it is 

 the instantaneous character of the separa- 

 tion which occasions the jar. 



The earthquake being thus a concomitant 

 of tectonic change, its regions of frequency 

 should be found in areas of diastrophic 

 activity, and its occurrence should be 

 rare and sporadic in areas of diastrophic 

 sluggishness. This corollary is so well 

 recognized that seismic activity is com- 

 monly regarded as the specific criterion 

 of relatively rapid crustal change. Other 

 criteria of such change are physiographic 

 and geologic, and these may be applied in 

 regions whose earthquake history is un- 

 known. They may also be used, in the 

 absence of seismic records, to give approxi- 

 mate indication of malloseismic localities. 



Bold and High Ranges. — It was pointed 

 out by Powell that, because erosion is 

 greatly stimulated by altitude and high 

 declivity, lofty mountains must be re- 

 garded as young; and under the principle 

 of continuity young mountains created by 



uplift are presumably still growing. They 

 are, therefore, phenomena of diastrophic 

 activity and presumably belong to mallo- 

 seismic districts. The conspicuous ex- 

 ample is Mt. St. Elias, which rises boldly 

 20,000 feet from its base, which was shown 

 by Russell to have continued its growth 

 during the life of the existing marine 

 fauna, and which recently has been signal- 

 ized by earthquakes of the first class. 



Fault Scarps.— Along the bases of block 

 mountains the lines of their limiting faults 

 are sometimes marked by fresh scarps 

 demonstrating recent increase of uplift. 

 In the Great Basin these scarps traverse 

 the alluvium of the piedmont slopes, a sur- 

 face of such simple type that their pres- 

 ence or absence can be observed with 

 confidence. Their absence suggests dias- 

 trophic inactivity or sluggishness, for 

 their effacement is a time-consuming 

 process. Their presence suggests dias- 

 trophic activity, and the suggestion is 

 strengthened when their relation to phe- 

 nomena of weathering and erosion is such 

 as to show that they were produced by a 

 series of recent uplifts instead of one only. 



Bifts. — A third physiographic criterion 

 is illustrated in California and was 

 brought to general attention by the San 

 Francisco earthquake. The slip causing 

 that shock occurred on the plane of a fault 

 which outcrops at the surface and has been 

 traced for hundreds of miles. The atti- 

 tude of the plane is vertical, but the dis- 

 placement along it was horizontal; and 

 there is reason to think that earlier move- 

 ments on the same plane were horizontal 

 also, for the fault does not separate a ridge 

 of uplift from a valley of depression but 

 traverses both valleys and mountains. At 

 all points it is included within a belt of 

 peculiar topographic habit, which the in- 

 vestigating geologists have designated as 

 "the rift." "Within this belt, which 

 ranges in width from a fraction of a mile 



