填空题

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)________________.