We __3__ do not know how the salt got into the ocean! We do know，of course，that salt is watersoluble (可溶于水的)，and so passes into the oceans with rainwater.The __4__ of the earth's surface is constantly being dissolved (溶解) and is passing into the ocean.

　　But we do not know whether this can __5__ the huge quantity of salt in oceans，if all the oceans were dried up，enough salt would be left to build a __6__ 180 miles high and a mile thick.Such a wall would reach once around the world at the Equator!

　　The __7__

　　salt that we all use is produced from seawater or the water of salt lakes，from salt springs and from

　　__8__ of rock salt.The concentration of salt in seawater __9__ from about three percent to three­and­one­half percent.The Dead Sea，which covers an area of about 340 square miles，__10__ about 11,600,000,000 tons of salt!

　　On the average，a gallon of seawater contains about a quarter of a pound of salt.The beds of rock salt that are found in various parts of the world were all __11__ formed by the evaporation (蒸发) of seawater millions of years ago.It is believed that the thick rock­salt deposits were __12__ after about nine­tenth of the volume of seawater had been evaporated.

　　Most Commercial salt is obtained from rock salt.The usual __13__ is to drill wells down to the salt beds.Pure water is pumped down through a __14__.The water dissolves the salt and it is forced through another pipe up to the __15__.

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