EFFECTS OF EARTHQUAKES 63 



nomena, which arise within and not below the crust of the earth. 

 The focus is usually situated at depths of eight or ten miles, while 

 depths of twenty-five to thirty miles are uncommon. 



The Effects of Earthquakes are geologically less important than 

 is usually supposed, and some of them which are commonly called 

 effects of earthquakes, are rather collateral results of the forces 

 which produced the earthquake. The heaving and wave-like roll- 

 ing of the ground, when violent, bring about great landslips in 

 mountain regions, and shake down enormous masses of earth and 

 rock from the cliffs and slopes, into the valleys beneath. Destruc- 

 tion of this character on a gigantic scale was a marked feature of 

 the great earthquakes which shook northwestern Greece in 1870. 

 These falling masses may temporarily or permanently block up the 

 mouth of a mountain valley, and by damming back the stream 

 which flows in the valley, convert it into a lake. The course of 

 rivers is not unfrequently altered by earthquakes, a ridge being 

 thrown up athwart the stream. Along the west coast of South 

 America are found many deserted stream channels, the streams 

 having evidently been diverted by the upheaval of ridges across 

 their courses. In a narrow valley such a ridge will act as a dam 

 and form a lake, but on a plain the stream will be diverted to 

 a new course. 



A very common accompaniment of violent earthquakes is the 

 opening of cracks and fissures in the ground ; these may close up 

 again after the shock has passed, but they often remain open as 

 yawning chasms. The two sides of such a fissure may remain 

 at the same relative level as before, or one side may be raised up 

 or dropped down, producing a dislocation or fault. The earth- 

 quake which originated in Owen's Valley, California, in 1872, and 

 was one of the most violent recorded within the United States, 

 was accompanied by a series of faults, running along the base of 

 the Sierra Nevada, and having a maximum displacement of twenty 

 feet. In May, 1887, an earthquake in Arizona and Sonora (Mex- 

 ico) was accompanied by a fault, which has been traced for thirty- 

 five miles, and has an average displacement of seven feet. Similar 

 phenomena have been recorded from many parts of the world. 



