Now, geologists naturally wish to complete this apparently incomplete chronology. For this reason they have repeatedly attempted to express their chronology in figures; for instance, to calculate how many thousand years it must have taken to form the separate strata, and how many thousand years have elapsed since their formation. These calculations have as yet, however, produced no certain result; the time is supposed to be some hundred thousands or millions of years, and we are told that these figures are only approximate guesses, and that no one can vouch for their correctness.1

2

At first the attempt to give in figures the duration of the most recent periods, and with them the approximate age of the human race, seemed to promise some success, and geologists have limited themselves to these periods; that is, according to the description given above to the post-pliocene or quaternary, and the recent periods. Geology here comes into contact with history, for the latter in inquiring into the earliest periods of the existence of the human race, must consider not only written documents, but also the traces of man which are found in the strata or on the surface of the earth; such as graves, old monuments and erections, implements, and sometimes even the remains of men, and of the animals with whom they came in contact. That portion of the inquiry into the earliest history of mankind which does not rest on documentary evidence, but on such geological and archæological investigations as I have described, is becoming in these days more and more a special scientific study, and is called 1 See vol. i. p. 290. 2 Ibid. p. 284.

Primæval History, Historical Anthropology, Archæological Geology, Human Palæontology, or Prehistoric Archæology. These investigations have called forth an almost boundless literature, especially since Lyell's book on the Antiquity of Man appeared in the year 1863, and they form at the present time one of the favourite occupations of real and amateur men of science and antiquarians.

The fact that these investigations into primæval history have a theological or religious interest has on the one hand tended to promote them, but on the other has had an unfavourable effect, inasmuch as it has affected the prejudices of several investigators. As I have shown in my last lecture, the Bible, as generally understood, states that the age of the human race is only a few thousand years. Now, geologists must wish either to see the Biblical chronology confirmed or disproved by their science, according to their religious position; and although it has been, perhaps fairly, asserted that a few older geologists, such as Deluc and Cuvier, have allowed their geological calculations to be influenced by the Biblical statements, and that it is for this reason. that they have calculated the antiquity of the human race at about 6000 years, there can be no doubt that some modern savants have taken pleasure in estimating the antiquity of the human race at many thousands of years, because they knew that they thus came into conflict with the Bible or the teaching of theologians, and with the belief of Christians.

I shall speak in the following lectures of Prehistoric Archæology. To-day I shall confine myself to geological investigations into the age of the human race.

Before geologists can calculate in figures the duration of any one of their periods, they must first of all have verified two things-1st, Some one effect which has been produced in this period by a definite cause; and 2nd, The measure of the effect which this cause produces in a definite time-a year or a century. A tree increases by one ring every year; if it is sawn through, and if the rings can be counted, its age can be exactly ascertained. Geology has not, of course, at its disposal such simple means of calculation, but men, of whom so far as I know Deluc was the first, have tried to obtain geological measures of time.'

The modern attempts which have been made to calculate in figures by means of geology the age of the human race, or to speak more accurately, the age of the human relics which have been found, may generally speaking be divided into two groups.

In the first place, human bones, implements, etc., have been found in different places in the earth, covered more or less deeply with loam, peat, river deposits, stalagmites, etc. These human remains, it is said, originally lay on the surface, and the layers have been gradually deposited upon them. If, therefore, we can calculate how long the latter have taken to form and be deposited, we shall know how long ago the bones and implements lay on the surface, and therefore approximately when the men who left these relics lived. But in order to calculate how many centuries were required for the deposit of these layers, we must know two things; first, the thickness of the layers, and secondly, how much is deposited in a century. If we know both,

1 Wiseman, Connection, etc. i. 366.

the age of the objects in question may be ascertained by a very simple calculation. The first may be found out by measurement; for instance, we know that human bones have been found under a layer of peat 30 feet thick and 40 feet below the sediment of the Nile. But the second equally important element in the calculation is, as we shall see, quite uncertain, because we can get no universally valid formula for the growth of peat and stalagmites, for the increase in river deposits. Therefore these geological formations will not do for a measure of time.

In the second place, human bones or implements have been found in places where, at the time of their deposit, the waters of the sea, of a lake, or a river, must have stood; whereas now either they have receded from thence, or the ground has been raised above their level. Thus in Scotland and in Sweden boats have been found sixty feet above the present level of the sea; in Switzerland lake dwellings at some distance from the present shores of the lakes; in the Somme valley in northern France flint implements and human bones, eighty to one hundred feet above the present river bed. The level of the sea, lake, or river has therefore been altered considerably. If we can find out how much time was necessary for such an alteration in the level and in the distribution of water and land, we shall be able to calculate the time at which those men lived who left these things. In order to calculate this, however, we must not only know how great this alteration is, which can be easily ascertained, but also how much of this alteration has taken place in each century. We shall see that the latter cannot be ascertained with certainty, and that

therefore we cannot by this means gain such a geological chronometer as I have described.

Many modern geologists who have made calculations in one of these two ways, have made the mistake of taking as the basis of their calculations either the slowest formations whose progress can be traced by observation, or averages taken from very few observations. But as Pfaff, Dawkins, and others have observed,' we ought not to trust to averages at all, because a geological change may take place very slowly in one place and at one time, and very quickly at another place and at another time; and it is unscientific to lay exclusive stress on the changes which take place slowly, as is usually done in most geological calculations of the age of the human race, because there can be no doubt that many important geological changes have taken place in a comparatively short time. G. H. v. Schubert* showed by several examples many years ago how deceptive are those chronological calculations which are founded on such changes.

When in the reign of the Emperor Francis I. the petrified trunk of a tree was found, the Emperor wished to know how long a trunk of like thickness must lie in the earth before it could be changed into a mass of stone. It occurred to the Viennese savants of that day that the Emperor Trajan had thrown a bridge over the Danube at Belgrade whose supports were still visible in the water. With the permission of the Turkish Court one of these supports was taken out and removed to Vienna. It was found that there was no change in the

1 Pfaff, Schöpfungsgeschichte, pp. 111, 126.

2 Die Urwelt und die Fixsterne, Dresden 1822, p. 279.