DAMAGE TO BRIDGES DURING EARTHQUAKES 653 



rigidity is incompetent to transmit the same stresses, and hence tends 

 to remain in status quo. In the mantle a slip over the basement 

 floor of rock will take the place of a more uniformly distributed ad- 

 justment within the floor. The valleys which intersect the mantle, 

 must, in consequence, reveal an amount of crustal contraction far in 

 excess of the average for the district (see Fig. 7, B). 



"Wherever rock valleys interrupt the surface of the shell beneath a 

 newer valley in the mantle of loose soil (see Fig. 8, ^), that portion of 

 the shell which lies outside the level of the bottom of the rock valley, 

 will by reason of the breaks in its continuity be less capable of trans- 

 mitting the compressive stresses. This may otherwise be expressed 

 by saying that this outer layer is not fully included in the pinch to 

 which the layer immediately below it is subjected. A consequent 

 reduction of its competency to transmit the earth's stresses will be 

 greatest for the immediate vicinity of the valleys, and hence the joints 

 will there be closed by a portion only of the average amount. The 

 walls of the rock valley must therefore tend to approach with the result 

 that they will push up the center of the river bed into a definite ridge, 

 and further give the effect of a settlement at the banks where abut- 

 ments are placed (see Fig. S, B). Next to the narrowing of the dis- 

 tance between banks and the accompanying parallel Assuring, these 

 changes are, as we have seen, the ones most frequently observed along 

 rivers after a destructive earthquake. 



University of Michigan 

 May 3, 1908. 



