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) element in man-made materials too— (27)
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) scheme of measurement, devised by the
Massachusetts Institute of Technology, grains qualify as sand if their
(29) is greater than 0.06 of a millimeter and less than 0.6 of a
millimeter. Depending on its age and (30) ,
particular sand can consist of tiny pebbles or porous granules. Its grains may
have the shape of (31) or spirals, their edges jagged or
smooth. They have come from the (32) of rocks, or from the
skeletons of marine organisms, which accumulate on the (33)
of the oceans, or even from volcanic eruptions.
Usually, the older the granule, (34) The fine, white
beaches of northern Scotland, for instance, are recycled from sandstone several
hundred million years old. (35) . Sand is an
irreplaceable industrial factor, which has many uses; but it bas one vital
function you might never even notice. Sand cushions our land from the sea’s
impact, (36) .