背诵篇章 37 Iceb cebergs are among nature's most spectacular creations, and yet most people have never seen one. A vague air of mystery Ivelops them. They come into being---- somewhere -----in faraway, frigid mid thunderous noise and splashing turbulence, which in most cases no one hears or sees. They exist only a short time and then slowly waste away just as unnoticed Objects of sheerest beauty they have been called. Appearing in an endless variety of shapes, they may be dazzlingly white, or they may be glassy blue, green or purple, tinted faintly of in darker hues. They are graceful, stately, inspiring ---in calm nd dangerous, and that they are --- h the night, in the fog and in storms. Even in clear weather one is wise to stay a safe distance away from them. Most of their bulk is hidden below the water, so their underwater parts may extend out far beyond the visible top. Also, they may roll over unexpectedly, churning the waters around them Icebergs are part of glaciers that break off, drift into the water, fioat about awhile, and finally melt Icebergs afloat today are made of snowflakes that have fallen over long ages of time. They embody snows that drifted down hundreds, or many thousands, or n some cases maybe a million years ago. The snows fell in polar regions and on cold mountains, where they melted only a little or not at all, and so collected to great depths over the years and centuries feathery points and become tiny grains of ice. When new snow fell on top of the old, it too turned to icy grains. So blankets of snow and ice grains mounted layer upon layer and were of such great thickness that the weight of the upper layers compressed the lower ones. With time and pressure from above, the many small ice grains joined and danged to larger crystals, and eventually the deeper crystals merged into a solid mass of ice. Topaz is a hard, transparent mineral. It is a compound of aluminum, silica, and fluorine. Gem topaz is valuable. Jewelers call this variety of the stone "precious topaz. The best-known precious topaz gems range in color from rich yellow to light brown or kish red. Topaz is one of the hardest gem minerals. In the mineral table of hardness it has a rating of 8, which means that a knife cannot cut it, and that topaz will scratch quartz. st of the world's topaz is white or blue. The white and blue crysta of topaz are large, often weighing thousands of carats. For ths reason, the value of topaz does not depend so mud on its size as it does with diamonds and many other precious stones, where the value increases about four times with each doubling of weight. The value of a topaz is largely determined by its quality. But color is also important: blue topaz, for instance, is often irradiated to deepen and improve its color. Blue topaz is often sold as aquamarine and a variety of brown quartz is widely sold as topaz. The quartz is much less brilliant and more plentiful than true topaz. Most of it is variety of amethyst: that heat has turned brown NOTE topa/ tupa;' topx/n(a)[U] transparent yellow mineral黄玉(矿物 (b)Ic] semi-precious gem cut from this黄玉;黄宝石. If the salinity of ocean waters is analyzed it is found to vary only slightly from place toplace. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity. One of these is the subtraction of water from the ocean by means of evaporation--- conversion of liquid water to water vapor In this manner the salinity is crystals of salt would be left The opposite of evaporation is precipitation, such as rain, by whid water is added to the ocean. Here the ocean is being diluted背 诵 篇 章 16 geomagnetic field that can be dated independently; the width of the stripes indicates the rate of the sea-floor spreading. 37 Icebergs Icebergs are among nature’s most spectacular creations, and yet most people have never seen one. A vague air of mystery envelops them. They come into being ----- somewhere ------in faraway, frigid waters, amid thunderous noise and splashing turbulence, which in most cases no one hears or sees. They exist only a short time and then slowly waste away just as unnoticed. Objects of sheerest beauty they have been called. Appearing in an endless variety of shapes, they may be dazzlingly white, or they may be glassy blue, green or purple, tinted faintly of in darker hues. They are graceful, stately, inspiring ----- in calm, sunlight seas. But they are also called frightening and dangerous, and that they are ---- in the night, in the fog, and in storms. Even in clear weather one is wise to stay a safe distance away from them. Most of their bulk is hidden below the water, so their underwater parts may extend out far beyond the visible top. Also, they may roll over unexpectedly, churning the waters around them. Icebergs are parts of glaciers that break off, drift into the water, float about awhile, and finally melt. Icebergs afloat today are made of snowflakes that have fallen over long ages of time. They embody snows that drifted down hundreds, or many thousands, or in some cases maybe a million years ago. The snows fell in polar regions and on cold mountains, where they melted only a little or not at all, and so collected to great depths over the years and centuries. As each year’s snow accumulation lay on the surface, evaporation and melting caused the snowflakes slowly to lose their feathery points and become tiny grains of ice. When new snow fell on top of the old, it too turned to icy grains. So blankets of snow and ice grains mounted layer upon layer and were of such great thickness that the weight of the upper layers compressed the lower ones. With time and pressure from above, the many small ice grains joined and changed to larger crystals, and eventually the deeper crystals merged into a solid mass of ice. 38 Topaz Topaz is a hard, transparent mineral. It is a compound of aluminum, silica, and fluorine. Gem topaz is valuable. Jewelers call this variety of the stone “precious topaz”. The best-known precious topaz gems range in color from rich yellow to light brown or pinkish red. Topaz is one of the hardest gem minerals. In the mineral table of hardness, it has a rating of 8, which means th at a knife cannot cut it, and that topaz will scratch quartz. The golden variety of precious topaz is quite uncommon. Most of the world’s topaz is white or blue. The white and blue crystals of topaz are large, often weighing thousands of carats. For this reason, the value of topaz does not depend so much on its size as it does with diamonds and many other precious stones, where the value increases about four times with each doubling of weight. The value of a topaz is largely determined by its quality. But color is also important: blue topaz, for instance, is often irradiated to deepen and improve its color. Blue topaz is often sold as aquamarine and a variety of brown quartz is widely sold as topaz. The quartz is much less brilliant and more plentiful than true topaz. Most of it is variety of amethyst: that heat has turned brown. NOTE: topaz / 'tэupжz; `topжz/ n (a) [U] transparent yellow mineral 黄玉（矿物）. (b) [C] semi-precious gem cut from this 黄玉; 黄宝石. 39 The Salinity of Ocean Waters If the salinity of ocean waters is analyzed, it is found to vary only slightly from place to place. Nevertheless, some of these small changes are important. There are three basic processes that cause a change in oceanic salinity. One of these is the subtraction of water from the ocean by means of evaporation--- conversion of liquid water to water vapor. In this manner the salinity is increased, since the salts stay behind. If this is carried to the extreme, of course, white crystals of salt would be left behind. The opposite of evaporation is precipitation, such as rain, by which water is added to the ocean. Here the ocean is being diluted