CRUSTAL MOVEMENTS 



187 



shore-lines are found about many lakes, and show that the surface 

 about the lake basins has suffered movement since the shore-lines 

 were formed. 



So widespread are the evidences of changes of level that it may 

 be said, with much probability, that more of the earth's surface has 



Fig. 182. Shore of former Lake Bonneville, Utah. (U. S. Geol. Surv.) 



been sinking or rising in recent times, than has been standing still. 

 This general statement seems to point to great instability of the 

 earth's crust; but it should be added that these changes go on, as 

 a rule, very slowly and quietly. The amount of movement is per- 

 haps a small fraction of an inch a year, more commonly than at a 

 faster rate. At times and places, however, the movements have 

 doubtless been more rapid; but even in these cases it is not to be 

 supposed that the movements were always violent. 



Ancient changes of level. Beaches and other features of ocean 

 shores are destroyed in time by erosion, if elevated above the water; 

 but there are still evidences of movements which took place so long 

 ago that no traces of shore-lines remain. Thus layers of rock, de- 

 posited as sediment (sand, mud, etc.) beneath the sea, are now 



