Since traces of the lowest forms of life have been found in practically the oldest known sedimentary strata, the problem of determining the age of life necessarily involves the determination of the age of those strata. But unravelling the dead past is not an easy task. One trying to unlock the “secrets of the cemetery of Nature’s dead,” walks on a shadowy road. His difficulties are many. It is like crossing a deep moat, climbing a steep wall.
Various methods have been applied to estimate the age of different periods of the earth’s history and much progress has been made toward a successful issue. Broadly, the procedure of different methods is the same. They do not differ in principle. “The rates of certain changes at the present day are determined as accurately as possible, and in imagination, the respective processes are traced backward in time until limiting conditions are arrived at.” Until the epoch-making discovery of radium, the two most outstanding methods used in calculating geologic time were (1) the rate of land erosion and deposition and (2) the rate of derivation of salt (sodium chloride) from the land and its accumulation in the oceans. Theoretically, it is simple to use the rate at which sediments are being deposited or solutions gathered into the ocean, as “geologic clocks” for estimating the length of past time. But in practice each method encounters its own difficulties and the results deduced give us at best only a rough idea of the immensity of time involved. I shall not deal with these individual estimates, but give the mean of several, which is 100,000,000 years, speaking roundly. As stated before, it is only a rough estimate. Nevertheless, it confirms the fact that the earth is very old—indeed much older than is commonly believed. In 1650, Bishop Ussher, in his interpretation of the “In the beginning” of Genesis, estimated that the earth was created 4004 years before the birth of Christ. According to this view the earth is 5931 years old today. Many cosmogonists and even some geologists of the 19th century held this Biblical interpretation to be the age of the earth. Other ancient religions held that the earth was created much earlier then 4004 B.C. Hutton, one of the founders of Geology, in his studies, found “no vestige of a beginning—no prospect of an end.” One cannot help sympathizing with Hutton. Whoever has made a trip to the Grand Canyon of the Colorado River, Arizona, must remember the awe-inspiring depth of the Paleozoic strata and thousands of feet of Proterozoic sediments beneath them ([ Pl. I]). If he has traveled farther north to the Cabinet Range, Montana, he must have carried with him an undying impression of the 35,000 ft. of the rocky monument, there built up by the Proterozoic seas.
LEAFLET 9.PLATE II.
COMPARISON OF AN ANCIENT AND A MODERN
MARINE ANIMAL OF ALLIED GROUPS.
1, A CAMBRIAN TRILOBITE (AFTER WALCOTT).
2, A MODERN HORSESHOE CRAB.
The present rate of denudation in the Hudson Bay region is one foot in about 47,000 years. How long then, has it taken the seas to lay down these miles of sediments, which are but a small fraction of the whole geologic column? Having considered all this, can we estimate, even approximately, the vast length of time that has elapsed between these periods of sedimentation, the so-called “gaps” or “breaks”—the torn, illegibly written pages of the history of the earth? Indeed, there are moments when all may feel that it is much beyond their comprehension. But, man, by nature, is at once humble and exalted. He is willing to admit his defeat, yet his thirst to conquer new knowledge, to know the truth, is never satiated.
The reason for the failure to arrive at an absolute result is not very far to seek. In computing geologic time, one has to calculate that which has elapsed, by some process in nature that takes place in one direction only and that does not change its rate when conditions alter. Whatever the method applied, be it the deposition of sediments, or the gathering of solutions, or the losing of heat by the sun and the earth, its rate of action should be uniform and uninterrupted. It should be independent of the changing conditions of the earth. Uniformity of the rate of action is the criterion for precise calculation. But we know that the past was quite different from the present. Different conditions have existed at different times during the earth’s history. The configuration of the earth, its climate, humidity, temperature and many other factors have varied from time to time and with them the rate of erosion, deposition and solution has either accelerated, diminished or ceased. The doctrine of uniformitarianism cannot be assumed in a changing world, even though our knowledge of the earth of the past can only be gained from a fuller study of the earth of the present.
When we consider the rate of sedimentation as a method for estimating geologic time, we take the total observed thickness of the geologic column, (estimated to be 70 miles) and divide it by the rate at which the sediments are now being laid down. But do we know this rate? By taking the average rate of sedimentation of nine large rivers[1] now in existence and assuming it to be the rate at which sediments were deposited in the past, an approximate conclusion can be arrived at. But it will be noticed from the figures of the rate of sedimentation of different rivers that they are widely variable, the highest being many times greater than the lowest. Can we then use an average of nine figures so out of proportion and yet expect a reliable quantitative value? Furthermore, the rate of deposition along the coast, near the mouth of a large and active river, is much higher than on a coastline where no rivers empty their sediments into the sea. The rate is also largely controlled by the character of the sediments deposited. Sandstone and shale are more rapidly deposited than limestone. Moreover, just as there is no knowledge of the duration of time of erosion between periods of sedimentation, there is also no record of the amount of detritus that has fallen off the edge of the continental shelf during widespread emergence of the continents.