MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 535 



of the collapse of caverns^ or other subterranean openings, the collapse 

 often causing the forcible ejection of water. Such openings are hkely 

 to be formed only where the surface material is incoherent. Sand- 

 stone dikes (p. 514) may perhaps be associated in origin with earth- 

 quakes. 



Earthquakes are likely to dislodge masses of rock in unstable posi- 

 tions, as on slopes or cliffs. They may also occasion slumps and land- 

 slides. ^ 



Effects on drainage. — The fracturing of the rock may interfere 

 with the movement of ground- water. After new cracks are developed, 

 or old ones opened or closed, the movement of ground-water adapts 

 itself to the new conditions. It follows that springs sometimes cease 

 to flow after an earthquake, while new ones break ouF where there 

 had been none before. The character of the water of springs is some- 

 times changed, presumably because it comes from different sources 

 after the earthquake. Joints may be so widened as to intercept rivulets, 

 and the waters thus intercepted may cause the further enlargement of 

 the opening. Illustrations of this sort are furnished by the earth- 

 quakes of the Mississippi valley (Lat. 36° to 38°) in 1811-12. Where 

 faults accompany earthquakes, they occasion ponds or falls where 

 they cross streams. Illustrations of both were furnished by the Ched- 

 rang River of India after the earthquake of 1897. ^ 



Effects on standing water. — Some of the most destructive effects 

 of earthquakes are felt along the borders of the sea. Thus the great 

 sea- wave of the Lisbon earthquake (1755) and that of the earthquake 

 which affected the coasts of Ecuador and Peru in 1868 are examples. 

 Such waves have been kno^^'n to advance on the land as walls of water 

 60 feet in height. They are most destructive along low coasts, for 

 here the water may sweep much more extensively o^^er the land. The 

 great loss of life during an earthquake has usualh^ been the direct 

 result of the great waves. Lakes are also affected by earthquakes, 

 their waters sometimes rising and falling for several hours aftei- the 

 initial disturbance, but lake-waves are much feebler than those of 

 the sea, and are not often destructive. 



Earthquake shocks are sometimes remarkably destructive to the 



1 Cross. TV^enty-first Ann. Rept., U. S. Geol. Surv., Pt. II, Chap. Y. 



2 Oldham. Report on the Indian Earthquake of June 12, 1897, p. 138. Mem. Geol. 

 Surv. of India. Cited b}^ Geikie, loc. cit., p. 374. 



