雅思写作经典范文 10 篇(五)
31 British Columbia
British Columbia is the third largest Canadian provinces, both in area and population. It is
nearly 1.5 times as large as Texas, and extends 800 miles (1,280km) north from the
United States border. It includes Canada’s entire west coast and the islands just off the coast.
Most of British Columbia is mountainous, with long rugged ranges running north and south.
Even the coastal islands are the remains ofa mountain range that existed thousands of
years ago. During the last Ice Age, this range was scoured by glaciers until most of it was
beneath the sea. Its peaks now show as islands scattered along the coast.
The southwestern coastal region has a humid mild marine climate. Sea winds that blow
inland from the west are warmed by a current of warm water that
flows through the Pacific
Ocean. As a result, winter temperatures average above freezing and summers are mild.
These warm western winds also carry moisture from the ocean. Inland from the coast, the winds from the Pacific meet the mountain barriers of the coastal
ranges and the Rocky Mountains. As they riseto cross the mountains, the winds are
cooled, and their moisture begins to fall as rain. On some of the western slopes almost
200 inches (500cm) of rain fall each year.
More than half of British Columbia is heavily forested. On mountain slopes that receive
plentiful rainfall, huge Douglas firs rise in towering columns. These forest giants often
grow to be as much as 300 feet (90m) tall, with diameters up to 10 feet (3m). More lumber
is produced from these trees than from any other kind of tree in North America. Hemlock,
red cedar, and balsam fir are among the other trees found in British Columbia.
32 Botany
Botany, the study of plants, occupies a peculiar position in the history of human
knowledge. For many thousands of years itwas the one field of awareness about which
humans had anything more than the vaguest of insights. It is impossible to know today just
what our Stone Age ancestors knew about plants,but form what we can observe of pre-
industrial societies that still exist a detailed learning of plants and their properties must be
extremely ancient. This is logical. Plants are the basis of the food pyramid for all living
things even for other plants. They have always been enormously important to the welfare
of people not only for food, but also for clothing, weapons, tools, dyes, medicines, shelter,
and a great many other purposes. Tribes living today in the jungles of the Amazon
recognize literally hundreds ofplants and know many properties of each. To them, botany,
as such, has no name and is probably not even recognized as a special branch of “knowledge” at all.
Unfortunately, the more industrialized we become the farther away we move from direct
contact with plants, and the less distinct our knowledge of botany grows. Yet everyone
comes unconsciously on an amazing amount ofbotanical knowledge, and few people will
fail to recognize a rose, an apple, or an orchid. When our Neolithic ancestors, living in the
Middle East about 10,000 years ago, discovered that certain grasses could be harvested
and their seeds planted for richer yields the next season the first great step in a new
association of plants and humans was taken.Grains were discovered and from them
flowed the marvel of agriculture: cultivated crops. From then on, humans would
increasingly take their living from the controlled production of a few plants, rather than
getting a little here and a little there from many varieties that grew wild- and the
accumulated knowledge of tensof thousands of years of experience and intimacy with
plants in the wild would begin to fade away. 33 Plankton
Scattered through the seas of the world are billions of tons of small plants and animals
called plankton. Most of these plants and animals are too small for the human eye to see.
They drift about lazily with the currents, providing a basic food for many larger animals.
Plankton has been described as the equivalent of the grasses that grow on the dry land
continents, and the comparison is an appropriateone. In potential food value, however,
plankton far outweighs that ofthe land grasses. One scientist has estimated that while
grasses of the world produce about 49 billion tons of valuable carbohydrates each year,
the sea’s plankton generates more than twice as much.
Despite its enormous food potential, little effect was made until recently to farm plankton
as we farm grasses on land. Now marine scientists have at last begun to study this
possibility, especially as the sea’s resources loom even more important as a means of
feeding an expanding world population.
No one yet has seriously suggested that “plankton-burgers” may soon become popular
around the world. As a possible farmed supplementary food source, however, plankton is
gaining considerable interest among marine scientists. One type of plankton that seems to have great harvest possibilities isa tiny shrimp-like
creature called krill. Growing to two or three inches long, krill provides the major food for
the great blue whale, the largest animal to ever inhabit the Earth. Realizing that this whale
may grow to 100 feet and weigh 150 tons at maturity, it is not surprising that each one 34 Raising Oysters
In the oysters were raised in much the sameway as dirt farmers raised tomatoes- by
transplanting them. First, farmers selected the oyster bed, cleared the bottom of old shells
and other debris, then scattered clean shells about. Next, they ”planted” fertilized oyster
eggs, which within two or three weeks hatched into larvae. The larvae drifted until they
attached themselves to the clean shells on the bottom. There they remained and in time
grew into baby oysters called seed or spat. The spat grew larger by drawing in seawater
from which they derived microscopic particles of food. Before long, farmers gathered the
baby oysters, transplanted them once more into another body of water to fatten them up.
Until recently the supply of wild oysters and those crudely farmed were more than enough
to satisfy people’s needs. But today the delectable seafood is no longer available in
abundance. The problem has become so serious that someoyster beds have vanished entirely.
Fortunately, as far back as the early 1900’s marine biologists realized that if new
measures were not taken, oysters would become extinct or at best a luxury food. So they
set up well-equipped hatcheries and went to work. But they did not have the proper
equipment or the skill to handle the eggs. They did not know when, what, and how to feed
the larvae. And they knew little about the predators that attack and eat baby oysters by the
millions. They failed, but they doggedly kept at it. Finally, in the 1940’s a significant breakthrough was made.
The marine biologists discovered that by raising the temperature of the water, they could
induce oysters to spawn not only in the summer but also in the fall, winter, and spring.
Later they developed a technique for feeding the larvae and
rearing them to spat. Going
still further, they succeeded in breeding new strains that were resistant to diseases, grew
faster and larger, and flourished in water ofdifferent salinities and temperatures. In
addition, the cultivated oysters tasted better! 35 Oil Refining
An important new industry, oil refining, grew after the Civil war. Crude oil, or petroleum - a
dark, thick ooze from the earth - had been known for hundreds of years, but little use had
ever been made of it. In the 1850’s Samuel M. Kier, a manufacturer in western
Pennsylvania, began collecting the oil from local seepages and refining it into kerosene.
Refining, like smelting, is a process of removing impurities from a raw material.
Kerosene was used to light lamps. It was a cheap substitute for whale oil, which was
becoming harder to get. Soon there was a large demand for kerosene. People began to
search for new supplies of petroleum.
The first oil well was drilled by E.L. Drake,a retired railroad conductor. In 1859 he began
drilling in Titusville, Pennsylvania. The whole venture seemed so impractical and foolish
that onlookers called it “Drake’s Folly”. But when he had drilled down about 70 feet (21
meters), Drake struck oil. His well began toyield 20 barrels of crude oil a day.
News of Drake’s success brought oil prospectors to the scene. By the early 1860’s these
wildcatters were drilling for “black gold” all over western Pennsylvania. The boom rivaled
the California gold rush of 1848 in its excitement and Wild West atmosphere. And it
brought far more wealth to the prospectors than any gold rush. Crude oil could be refined into many products. For some years kerosene continued to be
the principal one. It was sold in grocery stores and door-to-door. In the 1880’s refiners
learned how to make other petroleum products such as waxes and
lubricating oils.
Petroleum was not then used to make gasoline or heating oil. 36 Plate Tectonics and Sea-floor Spreading
The theory of plate tectonics describes the motions of the lithosphere, the comparatively
rigid outer layer of the Earth that includes all the crust and part of the underlying mantle.
The lithosphere is divided into a few dozen plates of various sizes and shapes, in general
the plates are in motion with respect toone another. A mid-ocean ridge is a boundary
between plates where new lithospheric materialis injected from below. As the plates
diverge from a mid-ocean ridge they slide on a more yielding layer at the base of the lithosphere.
Since the size of the Earth is essentially constant, new lithosphere can be created at the
mid-ocean ridges only if an equalamount of lithospheric material is consumed elsewhere.
The site of this destruction is another kind of plate boundary:
a seduction zone. There one
plate dives under the edge of another and is reincorporated into the mantle. Both kinds of
plate boundary are associated with fault systems, earthquakes and volcanism, but the
kinds of geologic activity observed at the two boundaries are quite different.
The idea of sea-floor spreading actually preceded the theory of plate tectonics. In its
original version, in the early 1960’s, it described the creation and destruction of the ocean
floor, but it did not specify rigid lithospheric plates. The hypothesis was substantiated soon
afterward by the discovery that periodic reversals of the Earth’s magnetic field are
recorded in the oceanic crust. As magma rises under the mid-ocean ridge, ferromagnetic
minerals in the magma become magnetized in the direction of the magma become
magnetized in the direction of the geomagnetic field. When the
magma cools and
solidifies, the direction and the polarity ofthe field are preserved in the magnetized
volcanic rock. Reversals of the field give rise to a series of magnetic stripes running
parallel to the axis of the rift. The oceanic crust thus serves as a magnetic tape recording
of the history of the 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 thunderousnoise 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 belowthe 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 ofsnowflakes 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 greatthickness that the weight of the upper
layers compressed the lower ones. With timeand pressure from above, the many small
ice grains joined and changed to larger crystals, and eventuallythe 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 thata 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 moreplentiful than true topaz. Most of it is
variety of amethyst: that heat has turned brown. 39 The Salinity of Ocean Waters
If the salinity of ocean waters isanalyzed, 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 crystalsof 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 so that the salinity is decreased. This may occur in
areas of high rainfall or in coastal regions where rivers flow
into the ocean. Thus salinity
may be increased by the subtraction of water by evaporation, or decreased by the addition
of fresh water by precipitation or runoff.
Normally, in tropical regions where the sun isvery strong, the ocean salinity is somewhat
higher than it is in other parts of the world where there is not as much evaporation.
Similarly, in coastal regions where rivers dilutethe sea, salinity is somewhat lower than in other oceanic areas.
A third process by which salinity may be altered is associated with the formation and
melting of sea ice. When sea water is frozen, the dissolved materials are left behind. In
this manner, sea water directly materials are leftbehind. In this manner, sea water directly
beneath freshly formed sea ice hasa higher salinity than it did before the ice appeared. Of
course, when this ice melts, it will tend to decrease the
salinity of the surrounding water.
In the Weddell Sea Antarctica, the densest water in the oceans is formed as a result of this
freezing process, which increasesthe salinity of cold water. This heavy water sinks and is
found in the deeper portions of the oceans of the world. 40 Cohesion-tension Theory
Atmospheric pressure can support a column ofwater up to 10 meters high. But plants can
move water much higher; the sequoia tree can pump water to its very top more than 100
meters above the ground. Until the end of the nineteenth century, the movement of water
in trees and other tall plants was a mystery.Some botanists hypothesized that the living
cells of plants acted as pumps. But many experiments demonstrated that the stems of
plants in which all the cells are killed can still move water to appreciable heights. Other
explanations for the movement of water inplants have been based on root pressure, a
push on the water from the roots at the bottom ofthe plant. But root pressure is not nearly
great enough to push water to the tops of tall trees. Furthermore, the conifers, which are
among the tallest trees, have unusually low root pressures. If water is not pumped to the top of a tall tree, and ifit is not pushed to the top of a tall tree,
then we may ask: how does it get there? According to the currently accepted
cohesion-tension theory, water is pulled there. The pull on a rising column of water in a
plant results from the evaporation of water at the top of the plant. As water is lost from the
surface of the leaves, a negative pressure, or tension, is created. The evaporated water is
replaced by water moving from inside the plant in unbroken columns that extend from the
top of a plant to its roots. The same forces that create surface tension in any sample of
water are responsible for the maintenance of these unbroken columns of water. When
water is confined in tubes of very small bore, the forces of cohesion (the attraction
between water molecules) are so great that the strength of a column of water compares
with the strength of a steel wire of the same diameter. This cohesive strength permits
columns of water to be pulled to great heights without being broken.
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