in a crystalline base partly of calcium carbonate, and partly of iron carbonate. These grains are oolitic in structure, and are probably the result of the same chemical change by which the calcareous beds of the Inferior Oolite in Lincolnshire have been converted into the iron ores. They occur, it will be noted, in several strata above the main bed, 12 feet in thickness in the above section.

This bed of iron ore is identical with that described by Blake and Hudleston at Abbotsbury in Dorset, where it occurs between the Kimmeridge clay above and the Corallian rocks below.

It is also physically identical with the valuable iron ore worked for many years at Westbury in Wiltshire, where it is met with at a lower horizon, being there separated from the Corallian limestones by 4 feet of marls and sands.

This stratum, although probably of purely local origin, is to be looked for in the beds above the Corallian throughout the whole of southern England, from Dorset eastwards. Its discovery at Dover is only second in importance to that of the South-eastern Coal-field. It will have to be taken into account in the future development of the coal-fields in southern England.

6. On the Cause of Earthquakes. By J. LOGAN LOBLEY, F.G.S.,
Professor of Physiography, City of London College.

Although a connection between the cause of volcanic and of seismic action is generally assumed, neither has been satisfactorily determined, though both are usually attributed in their inception to a shrinkage of the globe from secular cooling. The author took exception to this view, and adduced the great amount of rockfolding since the Cambrian Period, as showing that if this were due to planetary shrinkage, which rock-folding is assumed to prove, the earth's radial contraction must have been at least 100 miles during post-Cambrian ages, that such a contraction would require a loss of heat to the extent of 5,000° F., that therefore the globe would have had at the Cambrian Period a temperature 5,000° F. higher than at present, and that such a temperature was altogether incompatible with terrestrial conditions. Neither, apart from greater terrestrial heat, would meteorological conditions be at all like the present, with a surface 100 miles further from the centre, for an attenuated atmosphere and different gravities would affect all climatal conditions and all the agencies of nature.

Both the petrological and the palæontological teachings of the Cambrian rocks are entirely at variance with any such conditions, since they indicate terrestrial inorganic conditions and agencies similar to those of the present epoch.

It was submitted, therefore, that the assumption of a planetary shrinkage was opposed to geological facts, and that consequently another cause must be sought for seismic phenomena.

The author thought the hypothesis he brought before the Section in 1888 to account for volcanic action would meet the difficulty, and expressed the opinion that earthquakes were originated by chemical action arising from favouring physical conditions at separate and isolated dynamic foci, at moderate depths and quite unconnected with any central fused mass. These originating foci, like those of both volcanic and plutonic action, were in a thin outer rind of the globe of a few miles in thickness, which with all its foldings and plications rested upon a solid foundation, giving the earth its ascertained rigidity, and since the Cambrian Period there has been no appreciable decrease of the bulk of the globe or of terrestrial heat.

''On the Causes of Volcanic Action,' Report of the British Association for 1888 (Bath Meeting), p. 670.

7. On Certain Volcanic Subsidences in the North of Iceland.

By TEMPEST ANDERSON, M.D., B.Sc., F.G.S.

Perhaps the most striking features in Icelandic scenery are the giás (pronounced 'geow'), or fissures and chasms which are so frequently met with in all the districts in which recent volcanic activity manifests itself. They are usually, and in most cases rightly, ascribed to the lower stratum of a molten lava stream having obtained an outlet after the surface has consolidated into a crust of greater or less thickness. Giás of this class are, so far as the author has been able to observe, confined within the limits of a single lava stream, and do not affect previously formed rocks. Usually there is a large giá roughly parallel with each side of the original lava stream, and the space between these has subsided considerably. Any giás in this subsided portion are much smaller, and obviously of secondary importance. Examples of this are to be found in the well-known Almanagiá, at Thingvalla, which has a throw of about 100 feet, while the sides of the smaller giás which enclose the Logberg in the subsided portion are practically on the same level.

There are also several such subsidences near Lón and Ásbergi, in the North of Iceland. The main subsidence at Ásbergi is a little more complicated, though evidently due to the same causes. Here a large roughly triangular area has subsided, the throw at the apex being probably nearly 300 feet, but a space in the middle has remained at its original height, so that a depression has been produced like a great V, the portions both between and outside the legs having remained standing. In the case of Thingvalla it appears not unlikely that the lava which flowed down into the lake solidified on coming in contact with the water and formed a wall sufficiently strong to hold up the lava plain till it formed a firm crust, and that the giving way of this and the escape of the molten lower layers into the deeper parts of the lake caused the subsidence.

Similarly the lava which escaped from Ásbergi may have been that which now occupies the low ground near the estuary of the Jokulsá, in the direction of Lón.

On the east and south-east of Lake Myvatn a very extensive eruption, or series of eruptions, has taken place from a chain of craters locally called Gardr Borgir (the castles of Gardr,' which is the name of a farm). The lava flow has occupied nearly all the bed of Lake Myvatn, and flowed down the valley of the Laxá to its mouth at Laxamyri. All this stream of lava is very remarkable for the number and size of the spiracles with which it is studded, and a regular gradation of sizes exists, between spiracles the size of a haycock and cones some of which cannot be less than 200 feet high. These cones and craters, which constitute such a striking feature of Lake Myvatn, may probably be nothing more than spiracles formed by the escape of steam generated by the conflict of the hot lava with the water of the lake. The barrier which holds up the water of the present lake consists of this lava, and caves exist in it which are obviously channels by which molten lava has escaped. These and deeper-seated ones would be those by which the lava escaped and left the depression occupied by the present lake. Between the craters of eruption and the lake no spiracles were noticed, but there is a very remarkable series of rocks-the Dimmuborgir-masses of lava of fantastic shape, 30 or 40 feet high, which have remained standing while the intervening portions have subsided. They present slickenside marks where the subsiding portions have scratched the masses that have remained standing, and tide-marks where the crust has halted in its descent; also in many places bulgings, where the lava has been scarcely stiff enough to stand, and others where it has actually formed stalactitic masses. So far for actual lava subsidences.

The special object of this paper is to draw attention to a subsidence on the slopes of Leirnukr, a volcano several miles north of Myvatn, where a large strip of land, perhaps 200 yards wide and one mile or more long, has been let down to a varying depth, averaging perhaps 60 to 80 feet.

The faults bounding it, like nearly all the fissures in this district, run north and south; and the east face, which is most perfect, cuts right through a thick stream of old columnar lava and through a large boss of tuff, round and over which the lava has bedded itself, and also through the tuff rocks at each side of the lays

stream. It would appear worthy of consideration whether this great depression, which thus affects all the crust of the volcano impartially, may not have been caused by the falling in of one of the steam cavities which may be presumed to exist under volcanoes after the lavas have been expelled by the steam pressure.

This would accord with the observation that sedimentary rocks near volcanoes often dip towards those volcanoes. Mr. Goodchild has informed the author that the sedimentary rocks round Arthur's Seat are much thicker the nearer they are to that old volcano, as if the ground had slowly sunk while they were being deposited.

Near Lón the author was shown a small giá, said to have been formed during an earthquake in February 1885. The crack was of a freshness corresponding to such a date, and was only a few inches wide, and so short that it could not be determined whether it extended beyond one bed of lava. It certainly was not an example of the escape of liquid lava from below a crust, nor of a subsidence over a steam cavity, and its chief interest in this connection is as showing that at least three separate sets of causes are at work in producing the giás of Iceland.

FRIDAY, AUGUST 10.

1. A joint discussion with Section H on the Plateau Gravels, &c., West Kent, was opened by the following two communications :

(a) On the Geology_of_the Plateau Implements in Kent.'
By Professor T. RUPERT JONES, F.R.S., F.G.S.

This subject having been fully treated of by Professor Dr. Prestwich, the requisite references to his various memoirs elucidating the general geology of the local drift-deposits, the geological stages of their formation, and the peculiar flint implements of the plateau were given. He has shown that certain superficial soils on the North Downs between Sevenoaks and Rochester contain numerous rudely worked flints, discovered by Mr. B. Harrison; and that these were derived from a gravel, of very great antiquity, originally formed on the side of the old Wealden Hill-range or Mountain, which once rose about 3,000 feet above where Crowborough and other hills in Sussex now are. Man existed at the time of these gravels, and used the flints for tools. These gravels and the implements left in them were removed by natural agencies, such as rain, rivers, sea, frost, snow, and ice, and distributed by torrential streams on the Chalk slopes (now part of the North Downs) at a lower level on the flanks of the range.

These rude old flint implements have an ochreous colouring, due to ferruginous gravel whence they came; and are now found on the plateau, sometimes with limited patches of some of the ochreous flint gravel, together with Tertiary pebbles, less-worn flints, and fragments of Lower Greensand, on the red clay. with-flints' covering the Chalk. It was shown how desirable systematic excavations, to prove the extent and thickness of the implementiferous soil, would be.

Professor Prestwich's history of the origin of the ancient Wealden Dome, Island, and Hill-ranges, and of the gradual destruction of those uplands, in the course of untold ages, with the resulting formation and removal of successive geological groups of strata, such as the Thanet Sands, Woolwich-and-Reading Beds, London Clay, Lenham Beds, and the old ferruginous gravel with its rude implements above mentioned, was noticed in detail.

The Diestian or Lenham Beds were formed in the Early Pliocene period; and the denudation of Holmesdale probably began directly afterwards, at about the time of the Red or the Chillesford Crag in Late Pliocene, or in Post-Pliocene times; and the old ferruginous gravel had not only been formed, but washed away to a lower level before that time.

This paper has been printed in full in Natural Science, October 1894.

The ultimate denudation of the valleys cutting off the Chalk from the Weald being subsequent to the formation and removal of that gravel, the latter must have been Pre-glacial in age.

(b) On the Age of the Plateau Beds. By W. WHITAKER, F.R.S., F.G.S.

Mr. Whitaker said that the flints exhibited might be divided into two classes, the few that would be allowed by every qualified observer to be the work of man and the many as to the authenticity of which judgment should be deferred. He then alluded to the two points of view from which the subject was approached, the anthropological and the geological. In the former the work of man was the starting point; but, as a geologist, he thought that Nature should be duly considered, and the varied way in which she worked, sometimes leading to results that were somewhat unexpected. The district in question, too, was of a double character. In the first place, we had to do with a tract, south of the Chalk range, over which there were in parts beds of gravel, sometimes along the courses of the stream-valleys, but sometimes having no connection with the present drainagesystem. In the latter case the composition and position of the gravel seemed to point to a time when the features of the country were not the same as now, when the streams ran in different courses, and when the Chalk escarpment and other similar ranges of hill reached further south than now. He pointed out that the district was at and near the watershed between the Medway and the Darent, and that in such a position alterations in the flow of streams could be brought about by smaller causes than lower down along the river-valleys. The other part of the district was along and north of the great Chalk range, and the deposit here mostly met with over the Chalk could not properly be called Drift; it had not been brought into its present position from elsewhere by sea, river, or ice, but had grown where it stood; it was a residuum, the matter left from long-continued gradual dissolution of the chalk and the leaving behind of its flints and other insoluble matter, to which was added a mass of loam, resulting presumably from the remains of old Tertiary beds. With regard to the gravels, flint implements of undoubted workmanship having been found in them, it must be conceded that man existed at the time of the deposition of those gravels. This certainly carried man back, locally at all events, beyond the time of the river gravels, which occur in the bottoms and along the slopes of the valleys. He could not admit, however, that there was any good evidence to connect these ancient men with Pre-glacial or even with Glacial times, as there were no deposits of undoubted Glacial age in or near the district. Over the Clay-with-flints of the Chalk tract many implements had been found, but these were on the surface, and therefore we had no evidence of their age other than that given by their form. He understood that it had been said that a very few implements had been found in the Clay; but even were it so we should be little wiser as to their age, the formation of this clay having continued over a long time, right down to the present day. Elsewhere implements had been found in a brickearth that was associated with the Clay-with-flints; but in this case we were still ignorant of the age of the deposit, no other bed having been found above. He thought that in such a matter great caution was needed lest observers should be carried away by their zeal in discovery, and that the right spirit was to approach the question with wholesome doubt, contesting the views of those whose faith led them to believe very ordinary-looking chippings to be the work of design, so that they should have to prove that the balance of probabilities was in favour of their view.

:

The following Papers and Report were read:2. On the Traces of Two Rivers belonging to Tertiary Time in the Inner Hebrides. By Sir ARCHIBALD GEIKIE, F.R.S.

Many years ago the author had described part of the course of a stream which had cut its channel in the lava-plateau of the Isle of Eigg, and, as shown by the

materials of its gravel deposits, had flowed from east to west. The channel of the main river, as well as those of some of its tributaries, had been sealed up under a flow of pitchstone, which, after ages of waste, now forms, owing to its greater durability, the prominent ridge of the Scuir, the original higher ground of bedded basalt having been worn down into lower slopes. The river-course thus entombed must be assigned to the volcanic period of older Tertiary time in this country. Its western end is truncated by a precipitous sea-cliff, at the top of which a section of it is displayed, with its underlying shingle and overlying pitchstone, at a height of some 500 feet above the sea. This summer the author had enjoyed a favourable opportunity of visiting Hysgeir, a small low islet about eighteen miles to the west of Eigg, which had recently been identified by Professor Heddle as a continuation of the rock of the Scuir. He was able completely to corroborate this identification. The pitchstone of Hysgeir in its external forms and internal structure precisely resembles that of Eigg, presenting, indeed, so close a resemblance that it looks like a detached piece of the high ridge of the Scuir. Unfortunately, the columnar rock everywhere slips under the sea, and allows no trace to be seen of what it rests upon. If it be approximately as thick as it is in Eigg, its base may be 200 or 300 feet below sea-level. The gradual fall of the river-bed from east to west had been noticed at the Scuir, and the position of the pitchstone at Hysgeir showed a continued declivity in the same direction of perhaps as much as 35 feet in the mile. No visible rock rises to the surface of the sea between Hysgeir and Eigg. The region has been intensely glaciated, and the low ridge and rocky slopes of Hysgeir are strewn with erratics, which show that the ice moved westwards from the Inverness-shire highlands.

A much older river, but one still belonging to the volcanic period, has left some interesting records in the islands lying to the north of Hysgeir. A succession of coarse river-gravels are there found intercalated on different horizons among the bedded basalts. The materials of the lowest of these conglomerates are remarkably coarse, blocks 6 feet in length being occasionally visible. They consist in large measure of volcanic rocks, especially slaggy and amygdaloidal varieties. These constitute the largest and least water-worn blocks. Pieces of Torridon sandstone, epidotic grit, quartzite, and various granites and schists are generally well-rounded and smooth, and especially abound in the finer and more stratified gravels. The rapid dying out of thick sheets of coarse conglomerate is a conspicuous feature of the deposits, their place being sometimes taken by layers of fine tuff or volcanic mudstone, or by shales with remains of land-plants. Some portions of the conglomerate pass into true volcanic agglomerate, and this latter rock can in one place be seen to rise as a neck enclosing blocks of scoriæ and basalt sometimes 15 feet in length.

The sequence of events which these various deposits indicate appears to be as follows. During the outpouring of the lavas of the great basaltic plateaux of the Inner Hebrides a river flowed across the volcanic plain from the Western Highlands, whence it carried large quantities of shingle. By successive violent floods these materials, together with the detritus of the lava-fields, were strewn irregularly far and wide beyond the immediate channel of the river. In the pools left behind, fine volcanic silt gathered and entombed leaves and stems of the surrounding terrestrial vegetation. But volcanic activity still continued, and, though cones of slags and pumice were swept down, new eruptions took place by which masses of rock, sometimes 9 feet in diameter, were thrown out to a distance of a mile or more, and fresh streams of lava were poured out, completely burying the previous accumulations. Renewed river-floods of gradually lessening severity spread fine detritus over the cooled sheets of basalt, and again these later fluviatile deposits were entombed beneath fresh outbursts of lava. Perhaps no more striking evidence can be elsewhere obtained of the conditions of the land-surface over which, from many scattered vents, the materials of the volcanic plateaux of the Inner Hebrides were slowly piled up.