What is the absolute antiquity of the Palæocosmic age in Europe? We have no monumental or historical chronology to answer this question, but only the measures of time furnished by the accumulation of deposits, by the deposition of stalagmite, by the gradual extinction of animals, and by the erosion of valleys and other physical changes. These somewhat loose measures have been applied in various ways, but the tendency of geologists, from the prevalence of uniformitarian views, and the prejudice created by familiarity with the long times of previous geologic periods, has been to assign to them too great rather than too little value, both as measures of time and as indicating a remote antiquity.
With reference to the accumulation of deposits, whether derived from disintegration of the roof and walls of the cave, introduced by land floods or river inundations or by the residence of man, their rate is of very difficult estimation. Loose stones fallen from the roof, as in the case of Kent's Cave, would give a fair measure of time if we could be sure that the climate had continued uniform, and that there had been no violent earthquakes. Mr. Pengelly has, however, hopelessly given up this kind of evidence. Where, as in the case of many of these caves, land floods and river inundations have entered, these may have been frequent or separated by long intervals of time, and they may have been of great or small amount. Where, for instance, as in one of the Belgian caves, there are six beds of ossiferous mud, but for the fact that five layers of stalagmite separate them we might not have known whether they represent six annual inundations, or floods separated by many centuries from each other.
In the case of the Victoria Cave at Settle, Dawkins, reasoning from the accumulation of two feet of detritus over British remains that may be supposed to be 1200 years old, gives a basis which would at the same rate of deposit allow about 5000 years for the date of palæolithic men; but Prestwich and others, on the basis of stalagmite deposits, claim a vastly higher antiquity for the men who made the implements found in Kent's Hole and Brixham.
If we now turn to these stalagmite floors, when we consider that they have been formed by the slow solution of limestone by rain-water charged with carbonic acid, and the dropping of this water on the floor, and when we are told that in Kent's Cavern a marked date shows that the stalagmite has grown at the rate of only one twentieth of an inch since 1688, and that there are two beds of stalagmite, one of which is in some places twelve feet thick, we are impressed with the conviction of a vast antiquity. But when we are told by Dawkins that the rate of deposit in Ingleborough Cave may be estimated at a quarter of an inch per annum, and when we consider that the present rate of deposit in Kent's Hole is probably very different from what it was in the former condition of the country, stalagmite becomes a very unsafe measure of time. With respect again to the accumulation of kitchen-midden stuff in the course of the occupancy of caverns, this proceeds with great rapidity, when caves are steadily occupied and it is not the practice to cleanse out the débris of fires, food, and bedding. Even when the occupation is temporary, a tribe of savages engaged with the preparation of dried meat and pemmican in a very short time produce a considerable heap of bones and other rejectamenta.
Looking next to the extinction of animals, we find that the species found in the oldest deposits containing human remains are in part still extant. Others which are locally extinct we know existed in Europe until historical times, that is, within the last two thousand years. How long previously to this the others became extinct we have no certain means of knowing, though it seems probable that they disappeared gradually and successively. We have, however, farther to bear in mind the possibility of cataclysms or climatal changes which may have proved speedily fatal to many species over large areas. In any case we have this certain fact that, though the time elapsed has been sufficient for the extinction of many species, it does not seem to have sufficed to effect any noteworthy change on those that survived. Farther, we may consider that time is only one factor in this matter, and not the one which is the efficient cause of change, since we know no reason why one species of animal should not continue to be reproduced as long as another, but for the occurrence of physical changes of a prejudicial character.
We have still remaining the changes which have taken place in the erosion of valleys since the caverns were occupied. Dupont informs us that the openings of some of the caverns once flooded by rivers are now in limestone cliffs two hundred feet above the water, while no appreciable lowering of the bottoms of the ravines is taking place now. This would in some contingencies put back the period of filling of the caves to an indefinite antiquity. But then the questions occur—Was there once more water in the rivers or more obstruction at their outlets, or was the erosive power greater at one time than now, or were the river valleys excavated in still more ancient time, and partly filled with mud when the water entered the caves, and may this mud have been since swept away? So, in like manner, the waters flowing in the channels near Brixham Cave and Kent's Hole were apparently about seventy feet higher in times of flood than at present, but the time involved is subject to the same doubts as in the case of the Belgian caves. Hughes has well remarked that elevations of the land, by causing rivers to form waterfalls and cascades, which they cut back, may greatly accelerate the rate of erosion. Farther, there is the best reason to believe that in the glacial period many old valleys were filled with clay, and that the modern cutting consisted merely in the removal of this clay. Belt has shown in a recent paper [139] good reason to believe that this is the case with the Falls of Niagara, and that the cutting actually effected through rock within the later Pleistocene and modern period has been that only of the new gorge from the whirlpool to Queenstown, the main part of the ravine being of older date and merely re-excavated. This would greatly reduce the ordinary estimate of time based on the cutting of the Niagara gorge.
This leads us next to consider the occurrence of human remains and objects of art in the river-gravels themselves, and the amount of excavation and deposit involved in the deposition of these gravels. In the river-gravels of the Somme, and of many other rivers in France and Southern England, chipped flints and rude flint implements are found in so great quantity as to imply that the beds and banks of these streams were resorted to for flint material, and that the unfinished and rejected implements left in the holes and trenches, or on the heaps where the work was carried on, were afterward sorted by running water, perhaps in abnormal floods and debacles, such as occur in all river valleys occasionally, perhaps in that great diluvial catastrophe which seems to have terminated the residence of Palæocosmic man in Europe. Wilson has well shown how the heaps left by American tribes in and near their flint quarries would furnish the material for such accumulations. The time required for the erosion of the valleys and the deposit of the gravels has been very variously estimated. In the case of the Somme, which river is not appreciably deepening its bed, if we suppose it to have cut its wide valley to the depth of one hundred and fifty feet out of solid chalk since the so-called "high level" gravels of France and the South of England were deposited, the time required shades off into infinity. So Evans, in his work on "The Ancient Stone Implements of Great Britain," looking upon the amount of excavation of wide and deep valleys since the stone implements of Bournemouth are supposed to have been deposited in gravel, says, "Who can fully comprehend how immensely remote was the epoch when that vast bay was high and dry land?" and he becomes poetical in delineating the view that must have met the eyes of "palæolithic" man. And undoubtedly, if one is to be limited to the precise nature and amount of causes now at work in the district, the time must not only be "immensely remote," but illimitably so. The difficulty lies with the exaggerated uniformitarianism of the supposition that such causes could have produced the results. But, for reasons to be immediately stated, the time required is liable to numerous deductions; and recently Tylor, Pattison, Collard, and others have insisted ably on these deductions, as has also Professor Hughes, of Cambridge. I have myself urged them strongly in the work already referred to.
In the first place, when we see a deep river valley in which the present stream is doing an almost infinitesimal amount of deepening, we are not to infer that this represents all its work past and present. In times of unusual flood it may do in one week more than in many previous years. Farther, if there have been elevations or depressions of the land, when the land has been raised the cutting power has at once been enormously increased, and when depressed it has been diminished, or filling has taken the place of cutting. Again, if the climate in time past has been more extreme, or the amount of rainfall greater, the cutting action has then been proportionally rapid. Perhaps no influence is greater in this respect than that which is known to the colonists in Northeastern America as "ice-freshets," when in spring, before the ice has had time to disappear from the rivers, sudden thaws and rains produce great floods, which rushing down over the icy crust, or breaking and hurling its masses before them, work terrible havoc on the banks and alluvial flats, depositing great beds of gravel, and sweeping away immense masses that had lain undisturbed for centuries. Now we know that in Europe the human period was preceded by what has been termed the glacial age, and as it was passing away there must have been unexampled floods and ice-freshets, and a temporary "pluvial period," as it has been called, in which the volume of the rivers was immensely increased. Farther, it is an established fact that the period of the appearance of man was a time when the continents in the northern hemisphere were more elevated than at present, and when consequently the cutting action of rivers was at a maximum. This was again followed by a period of depression, accompanied probably by many local cataclysms, if not by a general deluge; and there are strong geological reasons to believe that this convulsion was connected with the disappearance from Europe of Palæocosmic man, and many of the animals his contemporaries. This view I advocated some time ago in my "Story of the Earth;" and more recently Mr. Pattison, in an able paper read before the Victoria Institute, has developed it in greater detail, and supported it by a great mass of geological authority. If the Palæocosmic period was one of continental elevation, when the greater seats of population were in the valleys of great rivers now covered by the German Ocean and the English Channel, and when the valleys of the Thames and the Somme were those of upland streams frequented by straggling parties and small tribes, and the seats of extensive flint factories for the supply of the plains below, and if this state of things was terminated by a diluvial debacle, we can account for all the phenomena of the drift implements without any extravagant estimate of time.
I quote with much pleasure on this subject the following from the report of a lecture on "Geological Measures of Time," by Professor Hughes, before the Royal Institution of London. Hughes was, like myself, a companion of Sir Charles Lyell in some of his journeys, though belonging to a younger generation of geologists, and is an accurate observer and reasoner.
"Another method of estimating the lapse of time is founded upon the supposed rate at which rivers scoop out their channels. Although no very exact estimates have been attempted, still the immense quantity of work that has been done, as compared with the slow rate at which a river is now excavating that same part of the valley, is often appealed to as a proof of a great lapse of time.