# Reading skills for academic study: Dealing with difficult words and sentences

### Exercise 3

Read the following text. Using the context given, try to work out the meaning and the grammatical structure of the word.

The Age of the Earth

The age of the earth has aroused the interest of scientists, clergy, and laymen. The first scientists to attack the problem were physicists, basing their estimates on assumptions that are not now generally accepted. G. H. Darwin calculated that 57 million years had elapsed since the moon was separated from the earth, and Lord Kelvin estimated that 20 - 40 million years were needed for the earth to cool from a molten condition to its present temperature. Although these estimates were much greater than the 6,000 years decided upon some two hundred years earlier from a Biblical study, geologists thought the earth was much older than 50 or 60 million years. In 1899 the physicist Joly calculated the age of the ocean from the amount of sodium contained in its waters. Sodium is dissolved from rocks during weathering and carried by streams to the ocean. Multiplying the volume of water in the ocean by the percentage of sodium in solution, the total amount of sodium in the ocean is determined as 16 quadrillion tons. Dividing this enormous quantity by the annual load of sodium contributed by streams gives the number of years required to deposit the sodium at the present rate. This calculation has been checked by Clark and by Knopi with the resulting figure in round numbers of 1,000,000,000 years for the age of the ocean. This is to be regarded as a minimum age for the earth, because all the sodium carried by streams is not now in the ocean and the rate of deposition has not been constant. The great beds of rock salt (sodium chloride), now stored as sedimentary rocks on land, were derived by evaporation of salt once in the ocean. The annual contribution of sodium by streams is higher at present than it was in past geological periods, for sodium is now released from sedimentary rocks more easily than it was from the silicates of igneous rocks before sedimentary beds of salt were common. Also, man mines and uses tons of salt that are added annually to the streams. These considerations indicate that the ocean and the earth have been in existence much longer than 1,000,000,000 years, but there is no quantitative method of deciding how much the figure should be increased.

Geologists have attempted to estimate the length of geologic time from the deposition of sedimentary rocks. This method of measuring time was recognized about 450 B.C. by the Greek historian Herodotus after observing deposition by the Nile and realizing that its delta was the result of repetitions of that process. Schuchert has assembled fifteen such estimates of the age of the earth ranging from 3 to 1,584 million years with the majority falling near 100 million years. These are based upon the known thicknesses of sedimentary rocks and the average time required to deposit one foot of sediment. The thicknesses as well as the rates of deposition used by geologists in making these estimates vary. Recently Schuchert has compiled for North America the known maximum thicknesses of sedimentary rocks deposited since the beginning of Cambrian time and found them to be 259,000 feet, about 50 miles. This thickness may be increased as other information accumulates, but the real difficulty with the method is to decide on a representative rate of deposition, because modern streams vary considerably in the amount of sediment deposited. In past geological periods the amount deposited may have varied even more, depending on the height of the continents above sea level, the kind of sediment transported, and other factors. But even if we knew exact values for the thickness of PreCambrian and PostCambrian rocks and for the average rate of deposition, the figure so obtained would not give us the full length of time involved. At many localities the rocks are separated by periods of erosion called unconformities, during which the continents stood so high that the products of erosion were carried beyond the limits of the present continents and "lost intervals" of unknown duration were recorded in the depositional record. It is also recognized that underwater breaks or diastems caused by solution due to acids in sea water and erosion by submarine currents may have reduced the original thickness of some formations. Geologists appreciated these limitations and hoped that a method would be discovered which would yield convincing evidence of the vast time recorded in rocks.