Explain and expand. • When a problem has proved intricable for more that 100 years, it is not likely to yield to more data collected in the old way and under the old rubric.

• When a problem has proved intricable for more that 100 years, it is not likely to yield to more data collected in the old way and under the old rubric.

Text 6

CORALS AND

PALAEONTOLOGICAL CLOCKS

The astronomers, geophysicists and other investigators whose con­cern is the origin and evolution of the earth are handicapped by a shortage of evidence. The events of interest to these workers occurred in times so distant that even geological records are seldom available. As a result the theories that have been advanced about such matters as the origin of the continents are largely conjectural. Moreover, as might be expected in the circumstances, the theories differ considerably and therefore are highly controversial.

An example of the kind of information that would help to overcome the handicap is a reliable measurement of the length of the day, that is, the speed of the earth's rotation on its axis. It is clear that the length of the day has increased slowly throughout geologic time; the earth's rotation has been slowed by the friction of the tides and may also have been changed slightly be internal processes. Hence the number of days in the year has decreased. If a "clock" could be found that had record­ed the days of ancient geological periods, it would be possible to arrive at a more precise measurement of a number of days in the year and so to obtain evidence about the earth's rotation and the factors affecting it.

Such a fossil clock may be at hand in certain corals. These organ­isms have long been known to have distinct bands that represent annu­al growth. The bands are themselves made up of narrower bands that seem to represent monthly growth and are probably related to the tides and monthly cycle of the moon. The intriguing possibility now under discussion is that the still finer ridges or bands found in some of the

corals represent daily growth. If this is the case, a coral that could be accurately assigned to a particular geological period (by radioactive dating or the evidence of stratigraphy) would provide a measurement of the number of days in the year at that time.

It was once taken for granted that the earth originated as a molten object and has gradually cooled. Mountains were thought to have formed through the consequent contraction of its interior, so the length of the day will shorten as the earth contracts and its mass moves toward its axis.

Since the discovery that rocks likely to be those making up the earth's interior contain radioactive elements, it has been proposed to assume that the earth's heat has been generated by radioactive decay and that the earth was originally cold. This theory fits in with modern ideas on the origin of the solar system, which is thought to have start­ed as a defuse cloud of gas and dust, in which the planets grew by accretion. Such an evolution would cause the day to shorten gradually, but by a much larger amount than on the basis that the earth began hot and has cooled.

There are some other hypotheses according to which the earth con­tracts and the length of the day shortens.

In contrast to these theories postulating a gradually shortening day, certain other theories assume that the earth has expanded and that the day has grown longer. The earth that has heated up will have expand­ed. The long ridges that run down the middle of several of the oceans offer some evidence that the earth has expanded and not contracted.

These various theories involve considerable disagreement over the length of the day in distant geological periods.

It seems clear that more observations on corals of different geolog­ical age will make it possible to determine the length of the day and

month throughout the geologic past. Those data in turn will yield im­portant information on the earthly history of the earth-moon system and may provide an important clue to their origin.

Corals may not be alone in this field. If other marine organisms have recorded time in the same say as the corals, we shall indeed have factual information on the early history of the earth.

Ш Give the Russian for:

to be handicapped by; to be available; to be conjectural; as might be expected in the circumstances; to be controversial, to overcome the handicap; to arrive at a more precise measurements; to be at hand; to be under discussion; to be assigned to; to be taken for granted; in contrast to these theories...; to provide an important clue to...

■ Give the English for:

недостаток доказательств; события, представляющие интерес для...; выдвигать теорию; значительно различаться; надежное измерение; на протяжении геологического времени; получать доказательства; в этом случае; если это имеет место; предлагать принять; соответствовать современным представлениям о...; значительное расхождение; относительно...; в свою очередь; получать важную информацию; таким же образом