admit of a doubt that those lakes, which are nothing but submerged valleys, are due in part to glacier action, although the other half of the causation on which they depend is to be sought in the subterranean action of subsidence.

In conclusion, I would observe that although the fact of a great subsidence and a reelevation of the land during the Glacial epoch has been generally admitted to be one of the facts of which there is the clearest evidence, it is nevertheless a fact of which all the conditions and all the consequences have been most imperfectly recognized.

Without venturing to go so far back as to imagine the process of subsidence and submergence, let us only think for a moment of that movement of reelevation which has certainly been one of the very latest of the great movements of geological change. If it took place very gradually or very slowly, it necessitates the supposition that every inch of our mountain-surfaces, up to at least 2000 feet, has been in succession exposed to the conditions of a sea-beach. Yet where are the marks upon them of such conditions ? We may suppose such marks to have been generally obliterated by later subaerial denudation. But against this is to be set the fact that the position and distribution of perched blocks and other erratics deposited by floating ice demonstrate, in my opinion, that very little indeed of such denudation has taken place since they were placed where we now see them. I could take any of you who are interested in this question to a precipitous hill near Inverary, some 1200 feet above the level of the sea, from the top of which you can look down on the masses of transported rock stranded upon its sides and base, precisely as one might look down from the top of some dangerous reef in the present ocean upon the débris of a whole navy of ships shattered upon it in some hurricane of yesterday. There they lie-some more or less scattered, some heaped upon and jammed against each other, with sharp angles and outlines wholly unworn, and, moreover, so distributed that you see at a glance their strict relation to the existing heights and hollows of the land, which must here have been the shoals and channels of the sea. These contours cannot have been materially changed since that sea was there. It seems that it must have been there, geologically speaking, only a very few days ago.

And this conclusion would seem to be confirmed when we observe the phenomena which are present in certain cases where the land has clearly rested for a considerable time and the ocean has left in raised beaches the evidence of its work at certain levels. Such raised beaches are to be found at many points all round our western coasts; but incomparably the finest and most instructive example of them is to be seen on the west coast of the island of Jura, near the mouth of Loch Tarbert, Jura, and extending for several miles to the north. These beaches are visible from a great distance, because their rolled pebbles are composed entirely of the hard white quartzite of the Jura mountains, which resists disintegration and is very unfavourable to the successful establishment of vegetation. I visited these beaches a few weeks ago, and, measuring the elevation roughly with a graduated aneroid, I found that they represent three more or less distinct stages of subsidence, one beach being about the level of 50 feet above the present sea, another about 75 feet, and a third at about 125 feet. Some others, which I saw only from a distance, appeared to be higher; and I believe, but am not quite sure, that further to the north they have been traced to the level of 160 feet.

But the feature connected with these sea-beaches, and especially with the lowest or the 50-feet beach, is the evidence it affords, first, of the length of time during which the ocean stood at that level, and secondly, and particularly, of the very recent date at which it must have stood there. As regards the length of time during which the ocean must have stood there, it is sufficient to observe the beautiful smoothness and roundness of the pebbles; they have been more thoroughly rolled and polished than the corresponding pebbles on the existing shores, equalling in this respect the famous pebble-beds of the Chesil Beach at Portland. Then, as regards the very recent date at which the ocean must have stood there, it is difficult to give in words an adequate idea of the impression which must be left on the mind of every one who looks at them. You see the curves left by the sweep of the surf, the summit level of its force, and the hollow behind that summit which is due to the exhausted crest-all as perfect as if it had been the

work of yesterday. Is is difficult to conceive how ordinary atmospheric agencies, and even the tread of sheep and cattle, should not have broken such an arrangement of loose material. But there are exceptionably favourable circumstances for the preservation of these beds. from absence of considerable streams and the protection of surrounding rocks. There is little or no evidence of glaciation anywhere around; and although it is certain that the sea which stood at those beaches so recently was a sea subject to glacial conditions, it is equally certain either that it continued to work there after those conditions had passed away, or, what is more probable, that that particular line of coast was protected from the drift of surrounding ice-floes.

If, now, we compare the evidence of recent action in these sea-beaches with the similar evidence connected with the position of erratics at far higher levels, which can only have been placed there by floating ice, I cannot help coming to the conclusion that the submergence and reelevation of the land to the extent of more than 2000 feet above the level of the present ocean has been one of the very latest changes in the history of this portion of the globe; and, moreover, that the reelevation has been comparatively rapid, probably by lifts or hitches of considerable extent, and that there were few, if any, pauses or rests comparable in duration with those recorded in the Jura beaches and in the cutting of the existing coasts.

Finally, let me repeat that whether this conclusion is correct or not (and I am well aware of the many difficulties which surround it), the general fact of submergence and reelevation is, perhaps, as certain as any conclusion of geological science, and that the consequences of it in accounting for the distribution of gravels and the most recent changes of denudation have never as yet been worked out with any thing approaching to consistency or completeness.

On the Sub- Wealden Exploration. By Major BEAUMONT, M.P.

On the Granite of Strath-Errick, Lough Ness. By JAMES BRYCE, LL.D. The author described a granite tract a little distance from the shores of Loch Ness, and near the Fall of Foyers. This fall took place originally over a cliff of Old Red Sandstone, and this stone being of a soft character gradually wore away until it formed a magnificent basin almost inaccessible at the bottom, and the action of the water had also worn the rock back to the slate which came between it and the granite. His attention had been called to the Loch-Ness granite tract by hearing that gold had been found in the lower valley of the Nairn, which passes through this granite district, and he supposed it probable that the gold might have its source in this granite tract in the same way as he had found the granite of Sutherland to be the true source of gold. After describing the limits of the granite tract, he pointed out a most remarkable circumstance connected with its history, which was illustrated by a section in a glen above Innerfaricaig. Here this triple granite rises from the valley in a direction sloping eastward, at first leaning against the Old Red Sandstone, and ultimately, further east, regularly overlying it, the metamorphism being very remarkable through about a foot of depth, and portions of granite being embedded in the Old Red. On the east side of the hill the Old Red Sandstone was regularly overlapped by granite, the strata of the Old Red dipping under it at an angle of 32 degrees. The well-known vitrified fort on the top of the hill contains both rocks highly vitrified. To the west of this another hill rises composed at the base of Old Red, and its upper part consisting of conglomerate granite. He called the attention of the Section specially to this conglomerate granite, and to the evidence which the whole district afforded that the granite here was truly irruptive, and not of that hydrothermal origin to which the granites further east have been ascribed. The only conglomerate granite similar to this with which he was acquainted was one that he had visited some years ago at Forkhill, county Armagh; and he called attention, especially of Professor Hull, to the connexions of these beds to the probable origin of a great irruption of granite.

On the Earthquake Districts of Scotland. By JAMES BRYCE, LL.D.

On the Tidal-Retardation Argument for the Age of the Earth. By JAMES CROLL, LL.D., F.R.S., of the Geological Survey of Scotland. Many years ago Sir William Thomson demonstrated from physical considerations that the views which then prevailed in regard to geological time and the age of our globe were perfectly erroneous. His two main arguments, as are well known, were, first, that based on the limit to the sun's possible age, and, secondly, that based on the secular cooling of the earth. More recently he has advanced a third argument, based on tidal retardation. It is well known that, owing to tidal retardation, the rate of the earth's rotation is slowly diminishing, and it is therefore evident that if we go back for many millions of years we reach a period when the earth must have been rotating much faster than now. Sir William's argument is, that had the earth solidified several hundred millions of years ago, the flattening at the poles and the bulging at the equator would have been much greater than we find them to be. Therefore, because the earth is so little flattened, it must have been rotating when it became solid at very nearly the same rate as at present. And as the rate of rotation is becoming slower and slower, it cannot have been so many millions of years back since solidification took place.

A few years ago I ventured to point out what appeared to be a very obvious objection to the argument, viz. that the influence of subaerial denudation in altering the form of the earth had been entirely overlooked; and as the validity of the objection, as far as I am aware, has never been questioned, I had been induced to believe that the argument referred to had been abandoned. But I find that Professor Tait, in his work on 'Recent Advances in Physical Science,' restates the argument as perfectly conclusive, and makes no reference whatever to my objection. As the subject is one of very considerable importance, I may be permitted again to direct attention to the objection in question, which briefly is as follows:

It has been proved by a method pointed out a few years ago, and which is now generally admitted to be reliable, that the rocky surface of our globe is being lowered on an average, by subaërial denudation, at the rate of about 1 foot in 6000 years. It follows as a consequence from the loss of centrifugal force resulting from the retardation of the earth's rotation, occasioned by the friction of the tidal wave, that the sea-level must be slowly sinking at the equator and rising at the poles. This of course tends to protect the polar regions and expose equatorial regions to subaerial denudation. Now it is perfectly obvious that unless the sea-level at the equator has, in consequence of tidal retardation, been sinking during past ages at a greater rate than 1 foot in 6000 years, it is physically impossible that the form of our globe could have been very much different from what it is at present, whatever may have been its form when it consolidated, because subaerial denudation would have lowered the equator as rapidly as the sea sank. But in equatorial regions the rate of denudation is no doubt much greater than 1 foot in 6000 years, because the rainfall is greater there than in the temperate regions. It has been shown in the papers above referred to that the rate at which a country is being lowered by subaerial denudation is mainly determined, not so much by the character of its rocks as by the sedimentary carrying-power of its river-systems. Consequently, other things being equal, the greater the rainfall the greater will be the rate of denudation.

We know that the basin of the Ganges, for example, is being lowered by denudation at the rate of about 1 foot in 2300 years, and this is probably not very far from the average rate at which the equatorial regions are being denuded. It is therefore evident that subaërial denudation is lowering the equator as rapidly as the sea-level is sinking from loss of rotation, and that, consequently, we cannot infer * Trans. Glasgow Geol. Soc. vol. iii. p. 1.

+ Nature,' August 21, 1871; 'Climate and Time,' p. 335.

Philosophical Magazine,' May 1868, pp. 378-384, February 1867, p. 130; 'Climate and Time,' chap. xx.; Trans. Glasgow Geol. Soc. vol. iii. p. 153.

from the present form of our globe what was its form when it solidified. In so far as tidal retardation can show to the contrary, its form, when solidification took place, may have been as oblate as that of the planet Jupiter.

There is another circumstance which must be taken into account. The lowering of the equator by the transference of the materials from the equator to higher latitudes must tend to increase the rate of rotation, or, more properly, it must tend to lessen the rate of tidal retardation.

On the Variation in Thickness of the Middle Coal Measures of the Wigan Coal-field. By C. E. DE RANCE, F.G.S., of H.M. Geological Survey.

From the Arley Mine, the lowest coal-seam of this series, to the Ince-Yard Coal, at Worthington, north of Wigan, the measures are 2200 feet in thickness, thinning 50 feet per mile to the S.W. to Prescot, where the measures are only 1445 feet in thickness, and 57 feet per mile to the N.E. towards Burnley, where the measures between the equivalents of these coals are only 1000 feet in thickness-proving the Wigan coal-basin to be not merely a synclinal of subsidence, but one of deposition, the axis of which was shown to have gradually travelled northwards in time from the district of St. Helens to a point north of Wigan. The importance of arranging colliery sections in a definite geographical direction, and the importance of noting the occurrence of very thin coal-seams and horizons of fire-clay and of seams full of Anthracosia, were insisted on as means of identifying equivalent coal-seams across a district. Great lateral shifts were shown to have occurred between many of the great N.N.W. faults which traverse the Wigan district and divide it up into a series of belts.

On Labyrinthodont Remains from the Upper Carboniferous (Gas-Coal)

of Bohemia. By Dr. ANTON FRITSCH.

The beds of gas-coal which are now being worked at so many localities, both in Europe and America, serve not only to illuminate our chambers, but to throw fresh light upon many branches of paleontological science; for these beds of gas-coal have been found to yield a remarkably fine fauna, especially rich in the remains of Labyrinthodonts, fishes, and insects."

During the last five years, I have been so fortunate as to discover in Bohemia two localities which afford us beautifully preserved relics of ancient life thus entombed in the gas-coals.

One of these localities is Nyran, near Pilsen, in the western part of Bohemia; other is Kounová, near Kakonitz, in the north-west of the country.

the

In both of these places the gas-coals are found to be situated on the top of the Coal-measures proper, but beneath the true Permian deposits. The plants of these beds are closely allied in character to those of the Coal-measures; but the animals appear to be of Permian types.

I do not intend upon the present occasion to enter into a full enumeration and description of these interesting fossils; but I take the liberty of submitting to this Section of the British Association series of specimens of casts and of plates of some of these fossils which I have brought to this country for the purpose of comparing them with the similar remains found in the British Coal-fields.

The first three plates exhibited contain enlarged drawings of very small Labyrinthodonts of the group called by Prof. Huxley Microsauria. One of these, not more than one inch (?) long, has the skeleton completely ossified.

The fourth and fifth plates are devoted to a large species of Labyrinthodon of about 5 feet in length.

Among the specimens, the author drew attention to the teeth of a Ctenodus, of which species the bony parts of the skull were found preserved.

Of the remarkable genus Diplodus a lower jaw with teeth served to show that these latter are not, as was formerly supposed, the dermal spines of a Ray.

Among the insect-remains was observed a new species of Gamsonychus, specimens of which cover the whole surface of some slabs of the rock. The restored

drawing illustrated the enlargement of the seventh pair of appendages in this species into swimming-feet.

The species of Julus, called by the author J. constans, shows how little the forms of this genus have changed in the interval between Palæozoic and recent times.

The rich materials which are now accumulated in the Museum of Prague will require for their illustration about 30 or 40 plates in the monograph which the author is now preparing on this very interesting vertebrate fauna.

On the Physical Geology and Geological Structure of Foula.
By GEORGE A. GIBSON, M.B., B.Sc. Edinb.

Taking up, in the first place, the physiographical geology of the island, in connexion with the agencies involved, the paper describes the coast scenery, and dwells upon the contrast between the low and rugged eastern side and the stupendous cliffs which overhang the western sea. This striking difference is shown to be due partly to the superior powers of resistance to weathering evinced by the materials of the sandstone rocks as contrasted with the crumbling schistose masses, and also to the fact that in the former the strata dip away from the cliffs.

The inland features are next taken up in detail. The five hills are found to be on the west side, three of them (Liorafield, the Sneug, and the Kame, which, taken by aneroid, reach 1000, 1250, and 1150 feet over sea-level) forming an axial chain, whilst the other two (Soberly, 650, and the Noup, 700 feet high) are distinct. The last three are noticed as forming precipices from their summits sheer into the sea, and all, except the Noup, which is dome-shaped, as having a conoid outline, the steepest sides of which face the north and east. As the dip is S.S. W., these hills are therefore held to have a contour in accordance with the empirical law, noted above, that the strata dip away from the side which has the steepest slope. The drainage is of course seen to be easterly.

The lithological character of the rock masses is then detailed, along with their architectural features and stratigraphical relations. The eastern side is described as composed of gneiss, very similar to that of Loch Maree, and of mica-schist with intruded veins of granite, having a strike from N.W. to S.E., agreeing therefore with the presumably Laurentian of Scotland; this, however, is not to be regarded as of great significance, on account of the variable strike of similar rocks in Shetland. The sandstone series is shown to be separated by a fault from the metamorphic rocks, running from N.N.W. by N. to S.S.E. by S., and along which dislocation the rocks are changed into hard quartz rock. There is described an unbroken succession of sandstones and flags for two miles, at an average dip of more than 25°, whose thickness cannot be estimated at less than 4400 feet.

No fossils having been discovered in these rocks, they are then compared in lithological character and position with the other sandstones of Shetland. Differences in composition and texture are pointed out to be due to altered conditions of deposition, the Bressay and Lerwick flags and sandstones belonging, with the Foula beds, to the Old Red Sandstone of the Caithness series.

On the Red Soil of India. By Dr. GILCHRIST.

On the Strata and Fossils between the Borrowdaile Series of the Coniston Flags of the North of England. By Prof. HARKNESS, F.R.S., and Prof. A. É. NICHOLSON, M.D.

On the Upper Limit of the essentially Marine Beds of the Carboniferous System of the British Isles, and the necessity for the establishment of a Middle Carboniferous Group. By Prof. EDWARD HULL, F.R.S., &c., Director of the Geological Survey of Ireland.

In this paper the author endeavours to show the equivalent stages throughout