THE WORK OF GROUND WATER 



65 



they usually contain a much greater quantity of dissolved minerals 

 than do streams. Silver Spring in Florida is carrying to the sea in 

 solution 340 pounds of mineral matter a minute, or 600 tons a day, and 

 it is estimated that, in central Florida, a little more than 400 tons of 

 rock a square mile are annually carried away in solution. This would 

 be equivalent to a lowering of the surface of the central peninsular 

 section of Florida by solution alone at a rate of one foot in five or six 

 thousand years. 



Falls Creek, Oklahoma (Fig. 71), receives water from springs which 

 contain much lime carbonate. In the immediate vicinity of the springs, 

 however, no deposits are formed, as there is a sufficient amount of 

 carbon dioxide present in the water to hold the lime in solution, but by 

 the time the stream has flowed a quarter of a mile large quantities 

 of carbon dioxide 

 have been given off, || 

 and travertine is de- 

 posited in the bed of 

 the stream in the 

 form of dams which 

 vary in height from a 

 few inches to 15 feet, 

 and are being built 

 up faster than the 

 stream can cut them 



Fig. 42. — Block diagram showing the formation of a 

 travertine terrace and natural bridge. Water containing 

 much lime carbonate emerges from springs in the lime- 

 stone at the right. Travertine has been rapidly de- 

 posited, forming the terrace and natural bridge. 



away. 



The great lime- 

 stone deposits at 

 Tivoli in Italy, from 

 which was quarried 



much of the stone used in the construction of the Coliseum and 

 St. Peter's at Rome and the interior of the Pennsylvania railroad 

 station in the City of New York, were laid down by springs. The 

 quantity and rapidity of the deposition of limestone under excep- 

 tionally favorable conditions is well shown in the great travertine 

 natural bridge at Pine, Arizona, more than 125 feet high, which, 

 together with a terrace of 25 acres, was formed by such a deposit 

 (Fig. 42). Springs containing lime carbonate or gypsum in solution 

 are called " hard," since, in washing, the fatty acids of the soap unite 

 with the dissolved minerals to form the insoluble " curd." 



By abstracting carbon dioxide from the water in which they grow 



