PRINCIPLES OF STRATIGRAPHY 



(Johnson-23.) During the earthquake of Yakutat Bay, Alaska, in 

 1899, great sections of the country moved as individual masses, 

 some blocks being elevated, some depressed, the extent of the dif- 

 ferential movement reaching 30 to 47 feet. Portions of the sea- 

 shore were elevated ; beaches only recently abandoned being now at 

 a considerable height above the sea, while many of the depressed 

 areas became submerged. Similar block movements occurred dur- 

 ing the great Icelandic earthquake of 1896. 



Disruptive Effects of Earthquakes. 



The more pronounced earthquake shocks often have a very de- 

 structive efifect on the rocks of the earth's surface. Land slips of 

 great extent may be set in motion by them. A piece of land one 

 mile long fell from the cliff at Scilla during the Messina earth- 

 quake. 



Darwin describes the overwhelming forces of the earthquake 

 of February 20, 1835, in Chile. He particularly notes the effects 

 on the islands in Concepcion Harbor, where upward of a hundred 

 villages were destroyed and where rocks from beneath the sea 

 were broken off and cast high up on the shore. "One of these 

 was six feet long, three broad, and two thick." Darwin describes 

 the destruction on Quiriquina Island as follows : "The ground in 

 many parts was fissured in north and south lines, perhaps caused 

 by the yielding of the parallel and steep sides of this narrow 

 island. Some of the fissures near the cliffs were a yard wide. 

 Many enormous masses had already fallen on the beach, and the 

 inhabitants thought that when the rains commenced far greater 

 slips would happen. The effect of the vibration on the hard 

 primary slate, which composes the foundation of the island, was 

 still more curious : the superficial parts of some narrow ridges 

 were as completely shivered as if they had been blasted by gun- 

 powder. This effect, which was rendered conspicuous by the fresh 

 fractures and displaced soil, must be confined to near the surface, 

 for otherwise there would not exist a block of solid rock throughout 

 Chile ... I believe this convulsion has been more effectual 

 in lessening the size of the island of Quiriquina than the ordinary 

 wear-and-tear of the sea and weather during the course of a whole 

 century." (Darwin-6, Chap. XIV.) 



Effects of Earthquakes on Topography. 



Many minor changes in topography in regions of frequent seis- 

 mic disturbances may have their origin in the earth tremors. 



