Sand covers not just sea-shores, but also ocean beds, deserts and mountains. It is one of the most common substances on earth. A

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 (26) major element in man-made materials too—(27) concrete 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 (28) accented scheme of measurement, devised by the Massachusetts Institute of Technology, grains qualify as sand if their (29) diameter is greater than 0.06 of a millimeter and less than 0.6 of a millimeter.
   Depending on its age and (30) origin, particular sand can consist of tiny pebbles or porous granules. Its grains may have the shape of (31) stars or spirals, their edges jagged or smooth; They have come from the (32) erosion of rocks, or from the skeletons of marine organisms, which accumulate on the (33) bottom of the oceans, or even from volcanic eruptions.
   Usually, the older the granule, (34) the finer they are and the smoother their edges. The fine, white beaches of northern Scotland, for instance, are recycled from sandstone several hundred million years old. (35) Perhaps they will be stone once more. in another few hundred million years.
   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, (36) and geologists say it often does a better lob of protecting, our shores than the most advanced coastal technology.