2016年职称英语理工a:2017职称英语理工B级：阅读判断练习(6)

副标题：2017职称英语理工B级：阅读判断练习(6)

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Where Has the Salt Come from?

　　Every now and then, we meet a fact about our earth that makes us feel strange and no answer for the fact has yet been found. Such a fact is the existence of salt in the oceans. How did it get there?

　　We simply do not know how the salt got into the ocean! We do know, of course, that salt is water-soluble, and so passes into the oceans with rainwater. The salt 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 explain the huge quantity of salt in oceans, if all the oceans were dried up, enough salt would be left to build a wall 180 miles high and a mile thick. Such a wall would reach once around the world at the Equator （赤道）!

　　The common salt that we all use is produced from seawater or the water of salt lakes, from salt springs （源泉） and from deposits of rock salt. The concentration （浓度） of salt in seawater ranges from about three per cent to three-and-one-half percent. The Dead Sea, which covers an area of about 340 square miles, contains about 11,600,000,000 tons of salt!

　　On the average, a gallon （加仑） 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 originally formed by the evaporation （蒸发） of seawater millions of years ago. It is believed that the thick rock-salt deposits were formed after about nine-tenth of the volume of seawater had been evaporated.

　　Most commercial salt is obtained from rock salt. The usual method is to drill wells （井） down to the salt beds. Pure "water is pumped down （抽进去） through a pipe. The water dissolves the salt and it is forced through another pipe up to the surface.

　　16 We have not fully understood how salt got into the ocean.