【正文】 ankton far outweighs that of the 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 “ planktonburgers” may soon bee 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 is a tiny shrimplike 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 devours more than one ton of krill daily.34 Raising OystersIn the oysters were raised in much the same way 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 bee so serious that some oyster 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 bee extinct or at best a luxury food. So they set up wellequipped 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 of different salinities and temperatures. In addition, the cultivated oysters tasted better! RefiningAn 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 being 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 . 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 to yield 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 doortodoor. 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. Tectonics and Seafloor SpreadingThe theory of plate tectonics describes the motions of the lithosphere, the paratively rigid outer layer of the Earth that includes all the crust and part of the underlying mantle. The lithosphere(n.[地]巖石圈)is divided into a few dozen plates of various sizes and shapes, in general the plates are in motion with respect to one another. A midocean ridge is a boundary between plates where new lithospheric material is injected from below. As the plates diverge from a midocean 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 midocean ridges only if an equal amount of lithospheric material is consumed elsewhere. The site of this destruction is another kind of plate boundary: a subduction 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 d