360 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



quently earthquakes of sensible strength would seldom or never 

 occur. In this way we may explain the comparative immunity of 

 many districts from earthquakes. 



Yet, when an even pressure thus arises, it may produce a steady 

 elevation of the land. Bending and warping of the strata may also 

 occur where the stresses are steadily applied, and under the cir- 

 cumstances the rocks would probably change their figure slowly 

 without snapping; when the earthquakes are more violent the rocks 

 are broken into smaller pieces and fault movements increase. 



§ 38. Oscillations of the strand shown in such phenomena as the 

 coal measures and fossil beds. 



No special effort is made in this paper to explain all the phe- 

 nomena of the earth's crust which offer difficulty to the geologist ; 

 and hence we have been chiefly concerned with phenomena of eleva- 

 tion. But ever since the time of Aristotle and Strabo it has been 

 justly remarked by sagacious observers that there has been not 

 only elevation of the land, but under certain conditions also subsi- 

 dence. Lyell and many other writers have discussed these oscillatory 

 movements which are well exhibited by the successive layers seen 

 in many of the coal measures. Some of these layers may be ex- 

 plained by the effects of damming up and drifting of vegetation to 

 places where it did not grow ; but even when allowance is made for 

 these causes, there still seems ample evidence of an oscillatory move- 

 ment of the land in many places. This is also well shown in fossil 

 beds, where sea shells often alternate with brackish water species. 



The expansion and contraction of the limits of the sea over large 

 areas of the low-lying shore is a frequent phenomenon, and a slow 

 oscillation of the strand seems the only rational explanation of it yet 

 offered. Such an oscillation is most easily explained by a sub- 

 stratum of fluid beneath the earth's crust, such as we show to exist. 

 Under just what conditions the land sinks is not clear. The insta- 

 bility may result from a number of causes of which the most probable 

 would seem to be : upward movement of neighboring regions, thus 

 weakening the support of the region in question and perhaps putting 

 additional load upon it, while the underlying fluid layer slowly 

 yields, thus causing subsidence. If during an earthquake a neigh- 

 boring area should be started upward, the strain would naturally 



