40 



PHYSIOGRAPHY 



Limestone sinks. The roofs of underground caves sometimes 

 fall in, making sink-holes at the surface. These are known as lime- 

 stone sinks (Fig. 25) . Such sinks are occasionally so numerous that 

 the surface about them is not cultivated, as in some parts of Ken- 

 tucky and Tennessee. From limestone sinks, tunnels often lead 





Fig. 25. A sink-hole of recent development near Meade, Kan. (From 

 photo, by Johnson, U. S. Geol. Surv.) 



down to caves. Some of these openings have been stopped up by 

 man, because cattle, going down into the sinks for the grass, occa- 

 sionally fell in. 



Dissolved mineral matter carried to sea. Much of the ground- 

 water finds its way to rivers after it seeps out, and the larger part 

 of the mineral matter in solution in rivers has come from ground- 

 water which has flowed to them. All the rivers of the earth are 

 estimated to carry nearly five billion tons of mineral matter to the 

 sea in solution each year. The transfer of so much mineral matter 

 in solution from the land to the sea lowers the land. 



Some of the mineral matter carried to the sea in this way remains 

 in the sea-water. Salt, for example ; is one of the substances carried 

 by rivers to the sea; and the larger part of all the salt ever carried 

 to the sea remains there, probably, to this day. On the other hand 

 much of the mineral matter carried to the sea is used by the animals 



