118 PHYSICAL GEOLOGY 



depressions called sinks. Sinks are often formed, however, 

 without the collapse of cavern roofs. Many are funnel- 

 shaped depressions dissolved in the rocks, through which 

 water runs down into a joint, which may lead to a cavern 

 (Fig. 110). If the two ends of a roof collapse, while the 

 middle portion remains, the latter constitutes a natural 

 bridge. Natural bridges are also formed in other ways 

 (p. 244). In Karst, on the eastern side of the Adriatic Sea, 

 the limestone rocks are so honeycombed by tunnels and 

 openings dissolved out by ground waters, that much of the 



FIG. 110. Diagram of caverns and sinks. 



drainage is underground. Large sinks abound, some of them 

 five or six hundred feet deep. Streamless valleys are com- 

 mon, and valleys containing streams often end abruptly 

 where the latter plunge into underground tunnels and caverns, 

 sometimes to reappear as great springs elsewhere. Irregular 

 topography of this kind, developed by the solution of surface 

 and ground waters, is known as karst topography, after the 

 type region in Austria-Hungary. 



Much of the material dissolved by the actively circulating 

 ground water of the upper zone is sooner or later carried to 

 the sea. It has been estimated that the rivers carry 

 4,975,000,000 tons of mineral matter to the sea in solution 

 yearly, and most of this is contributed by ground water. 

 It has also been calculated that the salt in solution in the 

 sea would, if it were taken out of solution and deposited, 

 form a layer 175 feet in thickness over the bottom, that 

 is, over nearly three fourths of the earth's surface. Most of 



