10 The Sea 
general swell and will recognize them as signs of danger 
ahead. 
A small island in the sea tends to bend swells as shown in 
the illustration. This curving of swells is observable some- 
times even when the island is beyond the horizon. A long, 
- confused line of turbulent waters extends from the oceanic 
island at right angles to the direction of the swell. The early 
Polynesian navigators are said to have made use of this line 
of confused water. They would simply locate such a line, fol- 
low it directly, and would of course be led to the island. At 
night this line of turbulence is often visible as a broad, phos- 
phorescent lane leading to the island. Similar modifications of 
wave-forms by land-masses will be met when we discuss the 
tides. 
SUBMARINE EARTHQUAKES 
Just as earthquakes occur on the land, they may occur on 
the ocean floor. Earthquakes are caused by the sudden re- 
lease of huge stresses built up in the rock layers of the earth's 
crust. The sudden release of these stresses results in a slipping 
of the rock layers (called faulting ) and violently shakes large 
areas of land or ocean bottom. The shock of the earthquake 
passes through the water at a speed of about 4500 feet per 
second. If the earthquake is a particularly violent one, the | 
shock will be felt on shipboard just as though the ship had 
run aground on submerged rocks. Professor H. V. Sverdrup, 
an internationally known student of the ocean, has suggested ~ 
that early navigators thought they had run aground when 
they felt such shocks. Old charts sometimes show “rocks” at 
places that we now know are thousands of feet deep. Sub- 
marine earthquakes have been known to rupture trans- 
oceanic cables. You may find it interesting to look through 
Sees? Afencs e 
