A proper interpretation of some geological phenomena requires that allowance shall be made for differential earth-movements that have taken place since the period of occurrence of the events or conditions under consideration.

Present differences of level in rocks of the same age may be partly due to actual differences in depth of the sea-floor on which they were deposited, but they may also be the results of subsequent differential earth-movements either of a regional or of a local character, the latter including ‘fauhs.’

In order to estimate the amount of displacement or differential movement, it is necessary to select a particular rock as a datum. The rock selected should combine, as far as possible, the following characteristics:—

1. It should be comparatively thin.

2. It should have a considerable horizontal extension.

3. It should combine similarity in physical characters and palæontological contents over a large area, so that uniformity of depth of deposit may be postulated.

To the north of Tainton, near Burford, Oxon, the 500 O.D. contourline passes through the old quarry grounds where the wellknown Great Oolite freestone may be seen dipping at a high angle to the north-east. The bank falls 100 feet to the brook, 400 yards distant, where culvert and pipe trenches for new field waterworks have exposed what may be a continuation of a fault marked on the one-inch geological map of the Ordnance Survey as following the course of Coombe Brook, a mile to the north.

Photography has now become so invaluable an accessory to every branch of science, that it is not surprising to find it has proved indispensable alike to the geologist and the palæontologist. One has but to turn back over the four decades of this Magazine and compare the number and quality of the illustrations, in the earlier with the later years, in order to realize the great advances that have been made by the application of photography both to process-plates and to illustrations in the text (especially in the last quarter of a century), which has so enriched and enhanced the value and interest of this and other scientific journals. That good half-tone process-blocks can be made from photographs of any geological subject taken in the field, is now well established; nevertheless, two points are absolutely essential to their success: (1) the photograph must be particularly clear and well-defined in its details; (2) the block, however good, may easily be rendered worthless by bad or careless printing; moreover, it must be reproduced upon paper with a finely finished surface, although it need not necessarily be the highly glazed kind (heavily loaded with kaolin) now so much in vogue for the illustrated Monthlies.

We may now examine how far these theoretical conclusions explain and are confirmed by what is seen in nature. First we know that in most formations there are great masses of what is now, or must have been once, fine-grained sediment. These often make up the bulk of the formation. We may quote the Ordovician, Silurian, and Devonian slates, the Keuper, Lias, Oxford and Kimmeridge Clays, the G-ault and London Clay; but we can give no such list of thick masses of marine sandstone. Fine sediment, therefore, has, as a matter of fact, made thicker masses of rock than coarse sediment; but this could not be the case if deposits thinned out seawards, where the fine sediment is carried. Again, it is impossible to imagine a thickness of a thousand feet and more constantly occupying a position near the shore; there is no room for it. If you depress the land, you remove the shore.

The existence of a raised beach formed, and probably elevated, before the deposition of the Boulder-clay has already been demonstrated in South Wales and Yorkshire. During the progress of the Drift Survey of the country surrounding Queenstown Harbour, a beach of similar age was observed along the shores of the harbour, and was subsequently traced at intervals along the adjoining coast of Waterford and Cork from Dungarvon to Clonakilty, a distance from east to west of about sixty miles.

Having been favoured by Mr. Walter Barratt, of Sunnybank, Newquay, Cornwall, with the opportunity of examining two Trilobites from the Devonian slate-rock—(1) from a cove near Trevose Head, (2) from the shore of the mainland opposite Trescore Island, Porthcothan, Cornwall,—I gladly avail myself of his permission to publish a note thereon.

In the Geological Magazine for October, 1902, Dr. Callaway mentions the explanation of these curves that I gave in a paper printed twenty-one years ago. His quotation should be read with the immediate context—”These [the tributary streams] keep open a channel into which the larger stream falls.” This is the essence of my argument.

Professor Phillips and Sir A. Geikie have remarked that an alluvial plain in the course of a river may be regarded as an old lake-bottom, now drained; the lake-like appearance being renewed in times of flood. Let us suppose ABCB (Fig. 1) to represent such an area with a river flowing through it in a straight line, and, on one side, a tributary stream, or, to use a shorter and more expressive term, an affluent, coming in at an angle. Such a condition, if it existed, would not continue even in consolidated alluvial soil; it is still less likely to have existed in the soft mud when the area was first drained. A succession of floods would certainly wash away the bank where the affluent, coming through it, had caused a break in its continuity. By this the river-bed opposite the affluent would be expanded beyond its requirements when at low water. At every flood the whole of the bed of the river and the adjoining area of land will be covered with water, the ordinary river channel being effaced. From this water suspended matter falls and forms a deposit, visible after the flood has subsided, but in greater quantity on the banks by the sides of the low-water channel than on the adjoining land.

The pleasant relations normally existing among geologists, biologists, and physicists have of late become a trifle strained on the question of the age of the earth. Biologists, having failed to induce either geologists or physicists to draw sufficiently large cheques on the bank of time, have taken to signing the same them-selves, adding the ciphers ad lib. Professor Poulton has ably championed the rights of the biologists to do so, and in the course of his argument he contends that there is evidence in the sedimentary strata to show that their rate of formation was not greater than that at which deposits are now being accumulated.

So far as I am aware, Professor Poulton's contention has not been either controverted or supported by any geologist. Hence it seems to be a suitable subject for discussion in the Geological Magazine.

In the first place, then, what would be the nature of the evidence we might a priori expect to find to show that one set of beds was accumulated in a shorter time than another of equal thickness ? Would there, in fact, be any difference such as would enable us positively to decide the question ?

Secondly, we may examine and compare rocks which we know, or have reason to suppose, have been formed at different rates. Now, according to Sir A. Geikie, if the rocks of the stratified systems were laid down at the greatest rate suggested by the facts of denudation. 73,000,000 years would be required; if at the least, 680,000,000.

In discussing a paper by Mr. J. S. Curtis on the Witwatersrand Gold Deposits, Mr. George Denny has raised the point that the slates or shales of the Witwatersrand, which especially characterise, but are not confined to, the Lower Witwatersrand Beds (Hospital Hill Series), are not of sedimentary, but of igneous origin. He relies chiefly on the fact that he has observed in places that a so-called band of slate cuts across the bedding of the quartzites. I think we are all prepared to agree with him that, where he can point out that this occurs, the rock which traverses the bedded formation must be an igneous intrusion; but such cases are rare. In the vast majority of sections where the slates are exposed, they are found to occur truly bedded, and in conformable relation with the quartzites with which they are associated. It seems to me that, after all, this is in the main a petrological question, which can be easily settled by the examination of the rocks in question under the microscope.

With this end in view I have examined a number of thin sections of these rocks, in all cases prepared from the cores of boreholes, on account of the difficulty of obtaining near the surface specimens sufficiently fresh and unweathered for microscopic examination; and I have selected geological horizons which are well known on the Witwatersrand. They are (1) the band of slates which occurs in the neighbourhood of the Bird Reef Series, and (2) the slates which occur in the footwall of the Main Reef itself, in both cases in the eastern portion of the Witwatersrand, as at Van Ryn and Geduld.

Mr. J. W. Knowles, M.R.I.A., of Ballymena, read a paper on what appeared to him to be flints chipped similarly to the eoliths, which he had found in the Interglacial gravels of Ireland. This gave rise to a discussion, in which five speakers in succession gave reasons, or rather expressed opinions, adverse to their artificial character; and as these seemed based on some misconceptions, and also as I was the only one who spoke for them, and was told I must be brief, and so could not fully reply to them, I do so in the present form, and I state, as far as I remember, what took place in as few words as possible.

Mr. Knowles himself took a neutral position, but suggested that, if they were implements at all, they might have been used as scrapers for scraping hæmatite.

The objections were: that these so-called eoliths were found in such extraordinary numbers that they might be the result of natural causes, and that their upholders must disprove this; that the flints in question were those of the Clay-with-Flints, a deposit due to the chemical dissolution of the Chalk-with-Flints; that, if admitted to be artificial, they could not be older than the Palæolithic implements of the high levels, as they were both found in the same deposits; that similar flints were to be found in the Boulder-clay, and, as man was post-Glacial, that was a positive refutation.

Mr. J. E. S. Moore's interesting work on the “Tanganyika Problem,” reviewed on p. 418 of the September number of the Geological Magazine, raises some interesting points in connection with African geology, though, as your reviewer rightly remarks, we are indebted to Mr. Moore rather for a statement of the problem “than for proposing an adequate solution of it.” At the same time it must be admitted that Mr. Moore's general geological conclusions appear to me, as a worker in Africa, much more in accordance with the facts as I see them in the course of my every-day life, than the hypotheses of certain eminent geologists who have preceded him. There is always a great disposition on the part of visitors to a little-known country to theorise, without having had time or opportunity for making those accurate and detailed observations which are essential if deductions are to be made which will stand the test of time. And the worst of it is, ‘authority’ has so much weight that local workers have the utmost difficulty in removing false impressions which may thus be caused. Two points to which I may refer are the “antiquity of the African continent” and the so-called ‘Rift-valleys.’ Now whatever may be the antiquity of Africa as a land surface, facts are constantly coming to light which throw great doubt on its having the stability attributed to it. Fossils of Jurassic and Cretaceous age are known from numerous places, and though these are chiefly confined to the coastal fringe, fresh discoveries are frequently increasing the area, such as the recent one far inland in the Sahara.

The following rapid and for most purposes sufficiently accurate method of estimating specific gravities of rocks, minerals, sands, etc., will recommend itself to those familiar with the apparatus of the chemical laboratory.