GEOLOGY and astronomy occupy, among the sciences, a peculiar position in this respect, that the geologist and the astronomer are interested in actual concrete points of time, whereas in other sciences we may study the course of events in time, but we do not as a rule inquire as to the particular concrete instant when a given event took place. In other words, in physics, for example, we may be interested in the length of time occupied by a given process, but we do not particularly care when the process occurred. The matter is different in astronomy and still more so in geology. The astronomer does not merely inquire at what velocities the various heavenly bodies move, but he is confronted with the problem: What was the position of a given heavenly body at a stated time? Or his problem may relate to the future, and he may inquire, for example, at what time eclipses of the sun will occur in the future. But the historical science par excellence is geology. The business of the geologist is to unravel the far remote past of the earth's history. This he has done by studying the rocks of which the earth's crust is composed, noting in particular the sequence in which the several strata have been laid down, one upon the other. To the uninitiated this might appear at first, quite a simple matter. But in point of fact the problem is much complicated through the vicissitudes which the earth's crust has undergone in the course of time. This crust is never wholly stationary but is undergoing a slow movement, the result probably of crust tides, similar to the tides in the sea, and also of other causes, such as the redistribution of material, owing to erosion and deposition by rivers. Perhaps the cooling of the earth also contributes its share in the remodeling of the earth's crust. However this may be, the fact is that the various strata of which this crust is composed do not everywhere lie simply in flat concentric spherical shells. Such an arrangement may occur here and there over restricted areas and restricted depths. But the rule rather than the exception is that the strata are thrown up into folds and sometimes turned bodily upside down. Thus the problem of determining their true sequence is anything but simple, and the geologist has developed a number of methods for attacking this problem. ' Of great assistance to him in the search after the earth's historic past are the fossils imbedded in the strata, which enable him, after he has studied the sequence of certain species in strata which show little or only obvious changes, to compare them with those appearing in other strata where conditions are doubtful, and to thus establish their natural sequence. The remarks made hitherto apply of course only to rocks deposited by sedimentation. Other rocks are formed by the injection of molten volcanic material through strata previously laid down. Here entirely different methods have to be employed to determine the age of the different constituents of the crust. It is not our purpose here to enter into a detailed discussion of the methods employed. It is merely intended to point out that the result of such methods as outlined above can only be to give us a rough idea of the order in which the several geologic epochs succeeded one another, without giving us more than at most an extremely crude idea of the length of these epochs in years. It is true that some estimate of this length of time might be gained from the thickness of the rocks laid down; tests in this direction have been made, but they have been rather unsatisfactory. Yet most geologists and every one interested in the results of geology naturally wishes to know how long the changes recorded in the rocks of which the earth's crust is composed actually occupied. And at the present day it is possible to form some sort of estimate in to this question. In this field as in so many others radium and radio-activity, which might at first sight be thought to have no relation whatever to this subject, has furnished a valuable clue to the inquirer. Some of the most recent conclusions in this field are cited by Prof. B. Hilber in a recent number of Die Umschau from which we reproduce in extract some of the principal points brought out. In many minerals containing uranium, radium is found as a product of disintegration of uranium. In such minerals there is a definite equilibrium or steady condition established, the uranium being converted at a certain definite rate into radium, which in turn is converted at the same rate into other produets and helium. Perhaps the expression “at the same rate” requires a little explanation. A given quantity of radium disintegrates very much faster than the same quantity of uranium would do, but when equilibrium is reached, a small quantity of radium is associated with such a large quantity of uranium, that the amount of urahium converted into radium is just equal to the amount of radium disintegrated per unit of time, so that the amounts of uranium and radium are (nearly) constant, changes involved being in reality excessively slow. Now the conditions of transformation of uranium into radium and helium are independent of all external circumstances. Assuming therefore that no helium escapes —an assumption which is fairly reasonable inasmuch as the total quantity of helium formed is very small—the amount of helium in a given rock must be a measure of the time which has elapsed since the birth of that particular rock. To be more exact, the time estimate just gained mU represent a minimum of the age of the mineral, the estimate being too short if any of the helium has escaped. In employing this method it is necessary to use as basis only minerals which are natural constituents of the strata in which they are found, that is to say, we must exclude any possible veins or intrusions formed in a stratum subsequent to the origination of the stratum itself. Now Strutt has determined experimentally the annual production of helium in thorianite and pitchblende. The absolute amount of helium present in a mineral depends of course not only on the age of the mineral but also on the absolute amount of radium and uranium present. In order to determine the age of a mineral by this method we must therefore consider the relative amount of helium. Following the terminology introduced by Strutt we shall speak of the “helium ratio” meaning thereby the number of cubic centimeters of helium per gramme of uranium oxide. The formation of one unit of this proportion of helium in a mineral requires eleven million years, which may therefore be regarded as our geological unit of time. Strutt gives the following figures as the result of his investigation: The time elapsed from the beginning of the quaternary to the present, 1,000,000 years; from a point not precisely defined in the oligocene period, 8.4 million; from the carboniferous, 31 million; from the lower part of the carboniferous, 150 million; from the archffian, 710 million. By the same method Schlundt and Moor have computed that since the glacial epoch twenty thousand years have elapsed. These figures have been criticised by Joly. He points out that if we divide a total thickness of sediments in the earth's crust by the 700 million years demanded by this theory, we obtain for the mean deposit a thickness two inches every four thousand years. This seems utterly inadequate to account for the actual thickness of the deposits as compared with what we know about the present rate of deposition. However this may be, other estimates seem to agree pretty well with those made on the basis of radio-activity. Thus, for instance, Mellard Reade has estimated upon geological grounds the time required for the formation of the earth's calcareous deposits, and has valued it at six hundred million years, a figure which agrees pretty well with Strutt's seven hundred million years. Again, the twenty thousand years demanded on the radio-active theory for the time elapsed since the glacial epoch agrees with the determination made by entirely different methods by a number of geologists who have made a special study of the subject. It is interesting now to inquire what light the conclusions thus reached throw upon the question of the rate at which the different species of plants and animals have been evolved. Summarizing the situation briefly, we might say that the flora and fauna with which we are acquainted at the present day is about fifty thousand years old. Some of the species now living are no doubt as much as one million years old. Most of the lower animals must have existed at an earlier period than this. According to Orbigny, the entire living world must have been completely replaced by a new species in all some twenty-seven times since the beginning of the world. (Lyell, on the other hand, places the estimate at twelve times.) Judging from the helium ratio in certain quaternary lavas in the Laacher Lake, 20 per cent of the crustaceans living to-day were in existence one million years ago. Assuming the same rate of change to have continued uniformly all the way back to the Eocene period, we arrive at the conclusion that 3 per cent of its crustaceans are living at the present day. As for the age of the human species, it would appear that the earliest man known, the Heidelberg man, lived about one million years ago. Men of the type such as we know him to-day, appeared on the scene considerably later.
This article was originally published with the title "The Length of Geological Epochs" in s , , 374 (March 2013)