The level at which these deposits began to be the rule, instead of the exception, was at about 120 feet above the Ordnance datum-line. Below this level they were spread out over all the flat or gently sloping ground, and only on the steeper gradients did not find a lodgment. As the water still further fell it drained into the old pre-diluvial smaller valleys, and in some cases the gravels have been swept out of these for some distance above the levels of the present brooks. The gravel and sand being all in motion together, there was a rough sorting of the materials, according to their weight; the largest and heaviest stones found their way to the bottom, whilst the sand was more generally distributed near and at the top of the gravel. Pre-diluvial man had left his stone-implements around his old settlements, and many of these were mixed up with the materials brought down from the west by the flood and deposited with them, the larger flint-implements sinking to the bottom along with stones of the same size. At still lower levels many of the implements were not moved far from where they had been left on the surface, and in some instances were not moved at all. Above the 40-feet contourline the bones of the large mammals that may have been lying on the surface deposits before the debacle, have been mostly swept away, excepting where preserved in hollows from the violence of the flood. A few of the large heavy teeth of the mammoth have been found in the gravels of the Ealing district, but the lighter bones are absent. In 1875, in digging gravel at the site of the New Museum of Natural History, at Kensington, the tooth of a mammoth was found at the base of the gravel, within 6 inches of the surface of the London Clay. No other bones occurred with it. A little farther west, in digging the foundations for houses in Cromwell Road, many fragments of bones belonging to the great ox, the red deer, and the mammoth were found near the top of the gravel, but none of the heavier teeth. It would appear from this that the remains were, like the stones of the gravel, deposited according to their specific gravity. Below the 40-feet contour-line, in the Brentford district, mammalian remains are most abundant. The reason why they should have been preserved there, is apparent when we look at a map showing the contour-lines of the district. To the west of Brentford, a spur of high land, all above 70 feet above the sea-level, comes down from Southall, as far as Hounslow. To the east of this the 50-feet contour-line runs to the north from Brentford, up past East Acton, towards Wormwood Scrubs, forming an inlet protected from the violence of the western flood. It is in this sheltered bay that the mammalian remains, so abundant between East Acton and Brentford, have been preserved. Mr. Alfred Tylor, some years ago, drew attention to the fact that all the mammaliferous deposits of the Thames Valley occur in situations similarly protected by escarpments to the westward; and my experience has been to the same effect. We have thus, on the one hand, deposits completely broken up by the violence of the flood, and, on the other, completely preserved from it. All the gradations between these two extremes should and do exist. In some the remains have been broken and mixed through the gravel, and even sorted according to their specific gravity. In others they have been drifted for a short distance, and the two mammalian faunas are mixed together without the bones being broken or much rolled. In others they have been only a little drifted from their original position; and in others, again, remain as they were first deposited, the bones of the same animal lying together. Much confusion must thus arise, and much difficulty in determining the succession of the beds. The difficulty is often increased when the remains lie at the junction of tributaries with the main stream. In such cases it has sometimes happened that the water filling the secondary valleys has been pounded back by the greater flood coming down the main one, so that, after the gravels have been spread out by the latter, the contents of the former have been washed over them. And thus, pre-diluvial mammaliferous sands lying in the tributary valleys, and protected from the violence of the flood coming down the principal channel, have, by the change in the direction of the currents during the falling of the water-level, become interstratified with-or even shifted unto the top of―much newer deposits. The only way to guard against being deceived by instances of this kind is not to accept any evidence of superposition as conclusive where the bones of the same animals do not lie near together, where the lamellibranchiate shells have not their two valves united, or where there are other signs of the deposit being a drifted one. In Norfolk and Suffolk the Middle Sands and Gravels. often contain fragments of marine shells derived from the Weybourne Sands and other beds. In Yorkshire they are known as the Hessle Sands and Gravels. B. Land Surface. How long the gap in the ice-barrier remained open after the discharge of the water of the first lake it is impossible to say. We have evidence, however, that it was long enough for some of the land-shells to occupy the surface again, and also, as the buried forest-bed at Ealing witnesses, for small trees to grow up. Until very recently I had not obtained this evidence, and was under the impression that the lake had been quickly re-formed. The forest-bed at Ealing shows clearly, however, that some years at least must have elapsed before this took place. I have found land-shells at Brentford, and near Bletchley, in Buckinghamshire, at this horizon. The Rev. H. M. De la Condamine described, in 1853, a deposit containing land and fresh-water shells on the top of the Middle Sands and Gravels, in the valley of the Ouse, between St. Ives and Huntingdon.* At Crayford there is a grey sandy clay containing shells of the fresh-water mollusks, Planorbis, Bythinia, Lymnea, and Anodon. The Anodon has the two valves united. The clay with these shells overlies the mammaliferous gravels and underlies the deposits that appear to be the representatives of the Upper Boulderclay, so that I think it must belong to this stage. It was probably at this time that the land shells contained in the Upper Loess and Diluvium multiplied greatly, and occupied the land, many of their natural enemies and competitors having been destroyed by the flood. It seems not impossible that remnants of paleolithic tribes or some of the great extinct mammals might have escaped destruction at the time of the rising of the waters of the first lake, and have spread over the area again before it was again submerged. I have, however, so far found no evidence whatever that they did so. The scarcity, if not absolute non-existence, of the bones of the species of deer, horse, and ox, that we know were not exterminated, makes it probable that the gap in the ice-barrier did not remain many years open. A. Upper Diluvium.-The Upper Diluvium, under which name I include the Upper Boulder-clay, the Upper Loess, the Hessle Clay, and the Upper Brick-clays, is the most widespread and the best preserved of all the glacial deposits. The gap in the ice-barrier had been closed and the lake re-formed, but this time the water was not suddenly discharged, and gradually cut out a channel between the Black Sea and the Mediterranean. There are several facts pointing to the probability that it was the channel of the * Quart. Journ. Geol. Soc., vol. ix., p. 271. Bosphorus that was thus formed, and not the Dardanelles. The deposits that were spread out during the continuance of the second lake were not subjected to the action of any sudden and tumultuous outpouring of the water, but only to its gradual subsidence. They have been but little denuded, and are still to be found nearly everywhere over the area that was submerged. During the rise, greatest extension, and subsidence of the water, sediments, varying greatly in character, were formed. The earliest of all in the Ealing district-the first sign of the second rise of the flood-is the bed of silt enveloping the stumps of the small trees of the buried forest. The similar silty clays at Brentford and elsewhere, with land shells, mark the same event. There are, in the alternations of gravel and sandy clay in some of the sections at Ealing, signs of small oscillations of the level of the water, during which the subangular gravels were partly denuded, and furnished some of the materials for the new deposits. Above this we have a bed of clay, showing a greater depth of water, and stones scattered through it, indicating the agency of floating ice. In addition to the single pebbles, and sometimes larger stones, there are patches of gravel and sand that appear to have been dropped in a frozen state into the bed of clay as it was forming. They are often angular, and show the lines of their original stratification, now lying at all angles or turned completely on end. They occur still more frequently in the Upper Boulder-clay of Norfolk and Suffolk than in the clay of the Thames Valley, and form one of the many evidences of the identity of the two deposits. The most probable explanation of their presence in the clay is, that the rising of the second lake took place wholly or partly in the winter season, and during an extreme frost; that ice was continually forming along the ever-widening shore, and, being broken up and floated off by the rising water, bearing masses of frozen gravel and sand with it; that thus a wide area of the lake was covered with graveland sand-laden ice, and that, on the melting of the latter, the frozen masses fell to the bottom and were imbedded in the clay. Very little detritus that can be traced to a northern source has been found south of the Thames, and this has led some geologists to hold the opinion that the country south of the river was not submerged when the Upper Boulder-clay was spread out. It is difficult to believe that this can have been the case, when we find the drift covering the northern brow of the valley. Prof. Prestwich has also observed transported rocks on the top of Well Hill, in Kent, at 600 feet above the sea. Along with large rolled flints were found a few fragments of chert and ragstone, which he refers to the Lower Greensand of the Sevenoaks range, some six miles further south, separated by the broad and deep valley of Holmsdale.* Mr. Whitaker has also seen lumps of handchalk, in Kent, at a high level, on Crocken Hill, eastward of St. Mary's Cray;t and Prof. Morris has informed me that there are patches of the loess, with its characteristic shells, preserved in fissures of the chalk at Bensted's Quarry, near Maidstone. We have an obvious explanation of the nearly total absence of northern drift south of the Thames in the theory that the Upper Diluvium was distributed by ice, floating over a great lake draining to the Mediterranean by way of the Black Sea; for the currents from the South of England would flow to the north of east, to get round the Hartz Mountains, and might be deflected still more to the north by those from the valley of the Rhine, and from the area of the English Channel. III. ON THE POSSIBILITY OF EXPLAINING PAST CHANGES IN THE UNIVERSE BY CAUSES AT PRESENT IN OPERATION. By S. TOLVER PRESTON. NY attempt to explain the phenomena of Nature by the recognised working of physical causation has been invariably welcomed. Thus the explanation of geological changes through the influence of time, by the recognised working of natural causes, is now generally accepted with satisfaction, and the idea of cataclysms or catastrophes has been abandoned. Might not the same thing apply to cosmical changes (or to changes in the Universe), or would not an attempt to explain them by the * Geological Magazine, October, 1874. |