THE OCEAN. 657 



supercrust, by the addition of sedimentary beds, has been attended by 

 the withdrawal of water from the oceanic and other superficial basins. 

 The metamorphism of strata has expelled this moisture, to a large 

 extent, from the beds thus altered, yet not wholly. The average 

 amount, in granyte, syenyte, porphyry, and all Archaean rocks, is not 

 over O06 per cent. ; while in other rock formations it may be 2*5 

 per cent. ; and in superficial clays and gravels it is at least 10 per 

 cent. 



If the thickness of the supercrust over the continental portion of 

 the globe averages five miles, and the average amount of moisture in 

 the formations, both metamorphic and unaltered, be 2*5 per cent., the 

 whole amount of water absorbed and confined would be a fortieth of 

 five miles, or about 650 feet in depth, for the area of the continents. 

 The deposits over the oceanic basins have relatively little thickness. 

 Whatever reasonable allowance be made for them, the whole loss to 

 the ocean waters, in depth, from this source, will not exceed 400 feet. 

 This confined water, while a feeble agent of change at the ordinary 

 temperature, is one of immense importance when much heat is present. 



As Delesse states, the water confined in terrestrial plants and. animals is another 

 part taken permanently from the oceans, since the commencement of Paleozoic time. 



The average thickness of the deposits, along the central portions of the Appalachian 

 region, has been estimated at seven miles. But, above the Archaean, that of the 

 region east of it is very thin; and west of it, to the Mississippi and for 600 miles 

 beyond, it will not average one mile. In the Rocky Mountains, there is a large crest 

 range, with no deposits above the Archaean : but, farther west, the mean thickness may 

 possibly be eight miles. To the north of the eastern United States, there is a very large 

 area of uncovered Archaean rocks. The mean thickness for the whole surface, there- 

 fore, will not exceed five miles. 



2. THE OCEAN. 

 1. Oceanic Forces. 



The ocean exerts mechanical force, by means of its — 



1. General system of currents. 



2. Tidal waves and currents. 



3. Wind-waves and currents. 



4. Earthquake-waves. 



The force of moving salt water is the same as for fresh water, ex- 

 cept the ditFerence arising from the greater density of the former, — 

 its specific gravity being one-thirty-fifth to one-fortieth more than that 

 of fresh water. 



The specific gravity of sea-water varies for different parts of the ocean. For the 

 waters of the southern ocean, it is 1-02919; the northern, 1-02757; equator, 1.02777; 

 Mediterannean Sea, 1-0293; Black Sea, 1-01418 (Marcet). In most seas receiving 

 large rivers, and in bays, the density is least. The specific gravity of the water of 

 East River, off New York City, at high tide, is 102038 (Beck). 

 42 



