34 



THE OPAL SEA 



Tidal 



waves. 



rock will not always withstand. The force of 

 water is almost incalculable. 



There is another wave occasionally seen on 

 the ocean that is called a " tidal wave " though 

 it has nothing whatever to do with the tide. 

 It is usually a wide, far-traveling undulation 

 set in motion by some shock to the sea basin 

 such as an earthquake. These subterranean 

 disturbances sometimes spread over a vast area 

 and set in motion waves that travel thousands 

 of miles with wonderful velocity. In 1877 one 

 of these waves started on the Peruvian coast of 

 South America, swept across the Pacific five 

 thousand miles to Hawaii, and even at that 

 distance maintained a rise and fall of some 

 thirty-six feet from trough to crest. This was 

 not so great as the earthquake wave of 1868 

 which, from the same region, traveled the Pa- 

 cific in a curved ring of perhaps eight thousand 

 miles in length — traveled at the rate of five 

 hundred miles an hour — and ran up on shores 

 ten thousand miles away with a breaker crest 

 thirty feet in height. 



The distance which waves will travel when 

 set in motion by violent disturbances is, again, 

 something almost incalculable. The explosion 

 of Krakatoa in 1883 produced ocean ridges one 

 hundred feet in height that rode over the 



Great 

 waves in 

 the Pacific. 



