共用题干 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 percent 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 theY 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.