650 DYNAMICAL GEOLOGY. 



just explained, may lead to a slide of the superincumbent beds 

 down the declivity. 



In 1806, a destructive slide of this kind took place on the Rossberg, near 

 Goldau, in Switzerland, which covered a region several square miles in area 

 with masses of conglomerate, and overwhelmed a number of villages. The thick 

 outer stratum of the mountain moved bodily downward, and finally broke up 

 and covered the country with ruins, while other portions were buried in the 

 half-liquid clay that had underlaid it and was the cause of the catastrophe. 



Similar subsidences of soil have taken place near Nice, on the Mediterranean. 

 On one occasion, the village of Roccabruna, with its castle, sunk, or rather slid 

 down, without disturbing or destroying the buiidings upon the surface. 



Besides (1) the transfer of rocks and earth, land-slides also cause 

 (2) a scratching or planing of slopes by the moving strata and 

 stones; (3) the burial of animal and vegetable life ; (4) the folding 

 or crumpling of the clayey layer subjected to the pressure, where 

 the effect does not go so far as to produce its extrusion and destruc- 

 tion. Such crumpled or folded beds of clay are not very uncom- 

 mon in alluvial regions (fig. 977). 



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 ratio between the velocity of salt water and its force is the same as 

 for fresh water (p. 635) ; but in the application of the ratio there is 

 a difference arising from the greater density of the former, — its 

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

 fresh water. Having determined the size of block that any given 

 velocity would be sufficient to transport, the size for other velo- 

 cities may be deduced by means of the ratio referred to. 



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

 the waters of the southern ocean, it is 1.02919; the northern, 1.02757; equator, 

 1.02777; Mediterranean Sea, 1.0293; Black Sea, 1.01413 (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 1.0203S (Beck). 



1. General system of currents. 



The system of oceanic currents is briefly explained on page 39. 

 It is part of the organic structure of the globe, irrespective of its 



