The distribution and characters of the beds are described. The succession compares very closely with that in the Ludlow district itself. The main differences are: (1) that the Aymestry Limestone is represented by mudstones west of the great fault-line, and (2) that all other divisions show greatly increased thicknesses.

There is no evidence of any stratigraphical break. On the contrary, the sequence is complete from the Lower Ludlow rocks up into the Old Red Sandstone, and the changes in lithology are usually quite gradual. The oncoming of the Old Red Sandstone conditions is discussed, with regard to their effect on the lithological and palæontological characters of the strata.

The extent of Old Red Sandstone, as indicated on present maps, must be greatly restricted, since most of the supposed Old Red Sandstone has been found to belong to the Temeside Group, which in this district attains a great development. The Silurian age of the beds in question is shown by the occurrence in them of Lingula minima, and of characteristic lamellibranchs, etc., also by comparison with similar strata in the Ludlow area.

A comparison with other districts in which Upper Silurian rocks are developed shows that deposition attained its maximum along the Welsh Border, the thickness of the formations decreasing rapidly southwards and eastwards.

On the east of the district-in the neighbourhood of the great fault-line the strata are considerably folded along axes ranging north-north-eastwards parallel to the main fault, with minor faults following the same direction. Away from the major faults the folding is gentler in character, and a series of folds ranging nearly due east and west make their appearance. Farther west the northnorth-eastward folding and fracturing reappear.

January 23rd.-Dr. Alfred Harker, F.R.S., President,
in the Chair.

The following communication was read:

‘On a Flaked Flint from the Red Crag.' By Professor William Johnson Sollas, M.A., Sc.D., LL.D., F.R.S., V.P.G.S.

The remarkable specimen forming the subject of the paper was obtained by Mr. Reid Moir from the base of the Red Crag exposed in the brick-pit worked by Messrs. Bolton & Company near Ipswich.

It is a fragment of a nodule of chalk-flint, irregularly rhombic in outline, with a nearly flat base and a rounded upper surface which retains the whitish weathered crust of the original nodule.

The base was formed by a natural fracture which exposes the fresh flint bordered by its weathered crust.

Both upper and under surfaces of the specimen are scored with scratches which are mainly straight, but in some cases curvilinear. Two adjacent sides have been flaked by a force acting from below upwards, in a manner that recalls Aurignacian or Neolithic workmanship. The two edges in which the flaked faces meet the base are marked by irregular minute and secondary chipping, such as might be produced by use. On the hypothesis that the flint has been flaked by design, these edges will correspond to the surface

d'utilisation' of M. Rutôt, and we should expect to find on the opposite edges of the flint the surface d'accommodation,' as in fact we do.

A singular feature, which seems difficult to reconcile with its use as an implement, is the restriction of the flaking on one edge to the weathered crust.

The origin of the flaking is discussed, and the author, while admitting that the fashioning of the flint is not inconsistent with intelligent design, concludes that the evidence is not sufficient to establish this beyond dispute. It is eminently a case of not proven.'

February 6th.-Dr. Alfred Harker, F.R.S., President,
in the Chair.

The following communication was read:

:

'Some Considerations arising from the Frequency of Earthquakes.' By Richard Dixon Oldham, F.R.S., F.G.S.

The publication of an abstract of twenty years' record of earthquakes in Italy gives an opportunity for studying the effect of the gravitational attraction of the sun; the period is so nearly coincident with the lunar cycles of 19 and 186 years that the effect of the moon may be regarded as eliminated, the record is of exceptional continuity and completeness, and the number of observations is large enough to allow of the extraction of groups sufficiently numerous to give good averages.

The distribution of the stresses is dealt with in text-books; there is a maximum upward stress, in diminution of the earth's attraction at its surface, at the two points where the sun is in the zenith or nadir, and a maximum downward stress along the great circle where it is on the horizon; but as, for the purpose of this investigation, a decrease of downward pressure is equivalent to an increase of upward, I shall take the line along which the downward stress is greatest as the zero-line, and express the amount of stress at any other time or place as a fraction of the difference between the net force of gravity along this line and at the point where the sun is in the zenith. The fraction, at any given time and place, depends solely on the zenith distance of the sun, which is continually varying with the revolution of the earth. At the equinox, when the sun is on the equator, the curve of variation between 6 A.M. and 6 P.M. is the same as in the other half of the day; at any other part of the year it is not symmetrical in the two halves of the day, but is the same during the day in the summer half of the year as during the night in the corresponding part of the winter half, when the declination of the sun is equal in amount, though opposite in

direction.

This gave the first suggestion for grouping the records. The year was divided into two halves by the equinoxes, and the day into two halves at six hours before or after noon, called day and night for convenience, irrespective of the time of sunrise or sunset. The result is given in the tabular statement below, the frequency *Boll. Soc. Sismol. Italiana, vol. xx. (1916) p. 30.

being expressed as a ratio to the mean, of each group, taken

From this statement it will be seen that the mean ratio of day to night shocks over the whole period is represented by the figures. 84: 116; for the summer half of the year they become 88: 112, and for the winter half 81: 119, showing that during the day the shocks are somewhat less frequent than the average in summer and somewhat more frequent in the winter, with an opposite variation during the night. Taken by itself this difference might be merely fortuitous, and further confirmation is required: this can be got in two ways. In the first place by comparison with other records, two of which, Milne's catalogue of Japanese earthquakes from 1885 to 1892*, and the aftershocks of the Indian earthquake of 1897+ stood ready for use. They show a variation identical in character with that of the Italian record. A second test depends on the argument that, if the variation is in any way seasonal, the divergence should be increased at the height of each season; the figures for the months of January-February and of June-July were taken out, as representing midwinter and midsummer respectively, and found to show a divergence in each case greater than, and in the same direction as, the respective half-years.

Taken by itself the variation, as between any pair of ratios, is as likely to be in one direction as in the other, but the odds against a complete concordance throughout the whole series is 31 to 1; there is, therefore, a strong presumption that the variations are not fortuitous, but due to some common cause which tends to increase the frequency during the day and decrease it during the night in summer, with the opposite in winter.

The variation in the frequency of earthquakes may, or may not, be connected with the variation in the gravitational stresses due to the sun; but there is another line of investigation by which a connexion may be better traced, dependent on the fact that the prevailing effect of the vertical stress is in the direction of lightening the load, and the prevailing direction of the horizontal stress between east and south, during the six hours before the meridian passages at noon and midnight, and of an increase in the *Seismol. Journ. Japan, vol. iv. (1895).

† Mem. Geol. Surv. India, vol. xxxv. pt. 2 (1903).

downward pressure and a horizontal stress between south and west during the next six hours. The record was accordingly grouped by the successive two-hour periods from XII to XII o'clock, and the mean amount of variation in the stresses was calculated for the The result is set forth in the appended tabular

same periods. statement:

DISTRIBUTION OF STRESSES AND SHOCKS IN TWO-HOUR PERIODS,
BEFORE AND AFTER MIDDAY AND MIDNIGHT.

From these figures it is seen that, while there is no apparent relation between the frequency and the total, or the horizontal, stress, there is a close one with the variation of the vertical stress ; the greatest number of earthquakes being in the period in which there is the greatest increase of downward pressure; as the rate of increase diminishes the number of shocks is less, suffering a further diminution as the pressure begins to decrease, and reaching its minimum in the period where the decrease in pressure is greatest, increasing again in the same way to the maximum.

An attempt to apply the same method to the Japan record gave a result which was, at first sight, contradictory and also inconsistent in itself, for it gave an absolute maximum at the time when the Italian gave a minimum, with another maximum, almost as great, in coincidence with the Italian; but, in any comparison, it is necessary to allow for the contrast in the character of the two records. The Italian does not contain more than two, or at most three, great earthquakes of the type that gives rise to long-distance records (bathyseisms), and the aftershocks account for no more than a quarter of the whole record; the Japanese record, on the other hand, is dominated by bathyseisms and aftershocks. Not only does the region give origin to an unusually large number of teleseisms, or bathyseisms, but aftershocks form fully three-quarters of the record, and nearly a half consists of aftershocks of the MinoOwari earthquake of October 28th, 1891. Taking these separately,

we get a curve of frequency similar to the Italian, except that the maximum and minimum are reversed, the greatest number of shocks corresponding to the period when the load is being lightened most rapidly, indicating that these shocks are due to a general movement of elevation rather than depression, a conclusion in accord with field observations of other great earthquakes. In addition, the shocks which occurred during the period 1885-90 were taken out, as representing a more normal activity, though still one in which aftershocks form fully half of the record, and the curve was found, as might have been expected from the character of the record, to combine the features of the Mino-Owari aftershocks with those of the Italian curve of frequency, of earthquakes prevailingly of the so-called tectonic' type.

These results are of twofold geological interest. In the first place they confirm the conclusion drawn from a study of the Californian earthquake of 1906*, that the great earthquakes differ from the ordinary, not merely in degree but in kind. They indicate that in the latter the main stress is compressive, probably due to settlement, and in the former to elevation or tension, a conclusion which is in accord with the fact that, in those cases in which it has been possible to compare accurate measurements made before and after the earthquake, the comparison has indicated an expansion, elevation, or both, of the area affected by the disturbance.

The second point of interest is that the figures give a means of estimating the rate of growth of the strain which produces earthquakes. If we accept the hypothesis that earthquakes, in the limited sense of their orchesis, are due to the relief by fracture of a growing strain when this has reached the breaking point, it can be easily shown that a variable strain, acting in alternate periods in increase or decrease of the general growth of strain, while leaving the average rate unaltered, will give rise to a corresponding variation in the frequency of shocks in each period; and, besides that, there is a simple relation between the magnitudes of the two stresses, to which the strains are due, and the variations from the mean frequency of earthquakes. A calculation on these lines shows that the growth of strain, for Italy, is such that, accepting the published estimates that an area of the earth's crust of the magnitude of Italy would crush under its own weight if left unsupported to the extent of 1/400 of the force of gravity, the breaking strain would be reached in about 3 years, starting from a condition of no strain. The aftershocks of the Mino-Owari earthquake give a little less than half this figure, which is again reduced to from five to six months if account is taken of the difference between the resistance of rock to tension and to compression. These figures are given for what they are worth; at the least, they are of interest as being the first authentic estimate which it has been possible to make of the time required to prepare for, and, thence, of the rate of growth of the particular tectonic process involved in the production of earthquakes.

* Q. J. G. S. vol. lxv. (1909) p. 14.