WATER. 655 



3. Under-currents. — The forcing of waters into bays, whether by- 

 regular winds or storms, causes a strong under-current outward, like- 

 that from the tides. This is especially marked when the entrance 

 of the bay is broad, so as to allow of an in-flow over a wide area, 

 while the deep-water channel is narrow. In some cases, ships lying 

 at anchor feel this under-current so strongly as to "tail out" the 

 harbor in the face of a gale which is blowing in. 



In the ordinary breaking of waves on a beach or in rocky coves, 

 there is an under-current (or under-tow) flowing outward along the 

 bottom. The wave advances and makes its plunge, and then its 

 waters flow back beneath those of the next wave, which is already 

 hastening on towards the beach. 



4. Earthquake-waves. 



In an earthquake, the movement of the earth may be either (1) 

 a simple vibration of a part of the earth's crust ; or (2) a vibration 

 with actual elevation or subsidence. In each case, the ocean-waves 

 which the earthquake, if submarine, may produce, have an actual 

 forward impulse, and are, therefore, forced or translation waves. They 

 have great power ; and, as there is no narrow limit to the amount of 

 elevation which may attend an earthquake, such a wave may be of 

 enormous height. An earthquake at Concepcion, Chili, set in 

 motion a wave that traversed the ocean to the Society and Navi- 

 gator Islands, 3000 and 4000 miles distant, and to the Hawaian 

 Islands, 6000 miles ; and on Hawaii it swept up the coast, tempo- 

 rarily deluging the village of Hilo. 



2. EFFECTS OF OCEANIC FOKCES. 



The effects of oceanic forces are here treated under the heads of — 

 (1) Erosion; (2) Transportation; (3) Distribution of Material, or 

 Marine and Fluvio-marine formations. 



1. Erosion. 



Erosion by currents. — But little erosion can be produced by the great 

 oceanic currents, on account of their slow rate of motion, and their 

 distance from the land. Still, the Labrador current, with its west- 

 ward tendency (p. 41), acting against the submerged border of the 

 continent, may have produced some results of this kina in past time, 

 if not doing so now. It has been supposed that the course of the 

 eteep outer slope of this submerged border (p. 12) has been deter- 



