Sand covers not just sea-shores, but also ocean beds, deserts and mountains. It is one of the most common substances on earth. And it is a (36)________element in man-made materials too — (37)________is largely sand, while glass is made by melting sand and some other elements.
What exactly is sand In fact, according to the most generally (38)________scheme of measurement, devised by the Massachusetts Institute of Technology, grains qualify as sand if their (39)________is greater than 0.06 of a millimeter and less than 0.6 of a millimeter.
Depending on its age and (40)________, particular sand can consist of tiny pebbles or porous granules. Its grains may have the shape of (41)________or spirals, their edges jagged or smooth. They have come from the (42)________of rocks, or from the skeletons of marine organisms, which accumulate on the (43)________of the oceans, or even from volcanic eruptions.
Usually, the older the granule, (44)________________. The fine, white beaches of northern Scotland, for instance, are recycled from sandstone several hundred million years old. (45)________________.
Sand is an irreplaceable industrial factor, which has many uses; but it has one vital function you might never even notice. Sand cushions our land from the sea’’s impact, (46)________________.
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and geologists say it often does a better job of protecting ......