Imatges de pÓgina
[ocr errors]

the requirements of the case, and most geologists consequently feel bound to seek light from the astronomer.

In a very suggestive address delivered last year by Mr. John #vans, as President of the Geological Society, this subject was discussed at some length. It is clear that if the position of the earth's poles could be shifted geographically, what is now polar land would be brought down into lower latitudes, and, provided the movement were sufficiently great, our difficulties would be at once got over. Most astronomers, however, following Laplace, have maintained that the position of the axis of the earth’s rotation is permanent. But Mr. Evans called attention to a paper, written nearly thirty years ago, by the late Sir J. W. Lubbock, in which the author pointed out the fallacy of some of the assumptions on which astronomers had based their conclusions. He held, indeed, that if from any cause the axis of rotation should not coincide with the axis of figure, the pole of the former would describe a spiral path around the pole of the latter, until the two at length coincided in position. Now geologists can show that the relative position of land and water has constantly been changed during the earth's history; such changes are, in fact, going on at the present day, the surface being upheaved here and depressed there, whilst solid matter is constantly being taken from one part of the surface and transferred to another. The shape of the earth must therefore be subject to variation, and the axis of figure consequently variable in position. But the axis of rotation always tends to coincide with the axis of figure ; if, therefore, the former be disturbed, the latter also becomes shifted. Mr. Evans suggested certain modifications in the disposition of land and water-modifications which, though large, were well within the range of geological probabilities—by which he believed that the axis of figure would be displaced 15° or 20° from its present position. Then, having got it into this new direction, it was further assumed that the axis of rotation must ultimately move into coincidence with it. Here, then, was a suggestion by which the difficulties of change of climate in the Arctic regions could easily be removed. It remained, however, for mathematicians to decide whether the position of the earth's poles could be thus easily shifted—to determine, in fact, what amount of displacement would result from the suggested alterations in the configuration of the earth's surface.

It is satisfactory to learn that the Rev. Professor Twisden has taken up Mr. Evans's suggestions, and patiently worked out the problem on the proposed data. He concludes, however, that the

1. Anniversary Address of the President, John Evans, Esq., F.R.S.' Quarterly Journal of the Geological Society, vol. xxxii. No. 126, May 1876, p. 53.

? « On Possible Displacements of the Earth's Axis of Figure produced by Elevations and Depressions of her Surface. By the Rev. J. F. Twisden, M.A. Abstracts of the Proceedings of the Geological Society, No. 331, February 21, 1877.

displacement would be so insignificant as not to exceed ten minutes of angle; and that, in order to produce as great a displacement of the earth's axis of figure as 20°, it would be necessary to assume that the elevations and depressions exceeded by many times the height of the highest existing mountains. Such a displacement of the axis of figure could only be effected, he believes, by a transference of matter equal at least to one-sixth of the whole equatorial bulge of the earth. But the transference of even this quantity of matter might take place without producing anything like the required displacement of the axis. Supposing, however, that a deviation of 20° could by any means be effected, the author holds that it would be followed by a sort of tidal movement in the ocean, so enormous that its greatest height would tend to be about twice the depth of the ocean. It will thus be seen that Professor Twisden's solution of the suggested problem is not very encouraging to geologists. Mr. Evans, however, has pointed out the necessity of treating the globe not as an absolutely solid spheroid, but as having its surface covered to a large extent with water; and not, perhaps, as a rigid solid, but rather as possessing to some extent plasticity or viscosity.

Possibly mathematicians may address themselves to the question in some modified form, from which results may be deduced more comforting to the geologist. It should not be forgotten, indeed, that the subject was ably discussed a few months ago by Mr. George H. Darwin,whose investigations, not being limited to a single definite problem, were of a general character, and whose results came much nearer to what geologists are seeking. In order to determine the amount of displacement of the earth's poles, it is necessary to ascertain the extent to which our globe may have suffered deformation by upheaval and subsidence during any one geological period. Mr. Darwin is led to conclude that from one-tenth to one-twentieth of the entire surface of the earth may from time to time have undergone elevation and subsidence, and that the greatest vertical amount of rise or fall may be equal to about 10,000 feet. If we suppose that one-twentieth of the earth's surface be elevated to this extent, and an equal area simultaneously depressed, the deflection of the pole will be 1° 46'; if the area of elevation be as great as one-tenth of the surface, the deviation will amount to 3° 17'. We may therefore conclude that a single geological change of large amount is competent, on certain assumed conditions, to produce an alteration in the position of the pole of from 1° to 3° of latitude. It will be understood, however, that this is the greatest possible result, obtained only under the most favourable conditions of the problem. If the earth be quite rigid, the redistribution of matter in the shape of new

: On the Influence of Geological Changes on the Earth's Axis of Rotation.' By George H. Darwin, M.A. Proceedings of the Royal Society, vol. xxv. No. 176, p. 328. See also Naturo, Feb. 22, 1877, p. 360.

continents could never cause a displacement of the pole from its initial position of more than 3o. But if the earth have the power of readjusting itself periodically to a new figure of equilibrium it is possible that the effect may be cumulative, and the pole may therefore have wandered as much as 10° or even 150 from its primitive position. During the original consolidation of the earth there must have been great instability in the geographical position of the poles.

In connection with this interesting subject it should be borne in mind that Sir William Thomson, at the Glasgow meeting of the British Association held last autumn, admitted it as highly probable that the earth’s axis of rotation may have been in early periods of geological history far distant from its present geographical position. The subject has also been recently discussed by the Rev. Professor Haughton, whose results, however, are not yet published.

It is always of interest to the philosophical geologist to note the existence of strata indicating a transition from one formation to another. These passage-beds, as the President of the Geological Society pointed out a few weeks ago in his anniversary address, are by no means to be regarded as curious anomalies, but rather as natural links in the chain of evidence as to the continuity of geological phenomena. In Bohemia geologists have long been disputing over the age of certain strata in their coal-fields, whether they are Carboniferous or Permian. Probably this question is best answered by not referring them definitely to either formation, but by regarding them as passage-beds from the true Carboniferous to the overlying Permian strata. The evidence on this subject has lately been laid before English readers by Dr. Ottokar Feistmantel, who is well qualified by a minute acquaintance with fossil botany to offer an opinion on the age of the plant-bearing beds.

Without entering into details which are of only local interest, we may remark that the coal deposits of Bohemia consist of two groups of strata, the lower of which is unquestionably Carboniferous, since it contains remains of both animals and plants which are recognised as true Carboniferous species. But the upper group of beds contains, in addition to seams of ordinary coal, a bituminous shale known as “gas-coal ;' and this shale is characterised by a fauna differing from that of the lower coals and suggesting Permian affinities, yet the associated plants are decidedly of Carboniferous types. The animal remains, consisting of amphibians, fishes, and arthropods, have been carefully studied by Dr. Fritsch, of Prague, whilst the associated plants were specially worked out by Dr. Feistmantel when

"Geological and Iistorical Notes on th Occurrence of a Fauna, chiefly of Permian Affinities, associated with a Carboniferous Flora in Gas-Coal in the uppermost Portion of the Bohemian Coal Strata.' By Ottokar Feistmantel, M.D. Geological Magazine (Trübner & Co.), March 1877, p. 105.

[ocr errors]

in Bohemia. It appears from the evidence of these naturalists that Carboniferous plants were contemporaneous with a Permian fauna, and that no strict line of demarcation can therefore be drawn, at least in the Bohemian coal-basins, between the true Carboniferous and the overlying Permian rocks. As the gas-coals thus form passage-beds between the two formations, they have been fitly termed Permo-Carboniferous. Such an association of a Carboniferous flora with a Permian fauna will remind palæontologists of the interesting commingling of organic remains in the famous Lignitic group of the Western Territories of America, where a Tertiary flora is found in company with a fauna of Cretaceous type. Every discovery which tends to bridge over a gap between two formations, and thus break through the old-fashioned notion of abrupt transition from one order of things to another, is a clear gain to the philosophy of geology, and as such deserves mention in these notes.

It is not long since Dr. Feistmantel was appointed to a post on the Geological Survey of India in succession to the late Dr. Stoliczka. Yet the new palæontologist has already managed to get through some good work in his special domain of fossil botany, and has addressed himself to one of the vexed questions in Indian geology-namely, the determination of the age of the great plant-bearing or coal series. If coal is found in a country, and found of good quality, it does not much matter commercially what its age may be, but scientifically the question is one of great interest. The age of the Indian coal-bearing beds and their correlation with the coal series of other countries are subjects which have frequently been discussed, one of the latest contributions to the discussion being a valuable paper by Mr. H. F. Blanford, read a short time ago before the Geological Society.

Mindful of the vagueness attaching to the expression “plant-bearing series,' Dr. Feistmantel prefers distinguishing the strata in question by Mr. Medlicott's term, the Gondwana system. The upper part of this series is divisible into two groups, the younger of which is referred to the Colites and the older to the Lias ; in other words, the Kachh and Jabalpúr series are of Oolitic, and the Rájmahal series of Liassic age. The lower part of the Gondwana beds is likewise separable into two groups—the upper or Panchét, and the lower or Dámúdá seriesboth of which, according to the author, may be referred to the Trias; the Panchét group belonging probably to the Keuper, and the Dámúdá group to the Bunter. It is the Dámúdá beds which contain


5 Kurze Bemerkungen über das Alter der sog. älteren kohlführenden Schichten in Indien.' Von Dr. Ottokar Feistmantel, in Calcutta. Neues Jahrbuch für Mino. ralogie u.8.9., 1877, Heft ii. p. 147. See also Geolog. Mag. Nov. 1876.

• On the Age and Correlations of the Plant-bearing Series of India, and the former Existence of an Indo-Oceanic Continent.' By Henry F. Blanford, Esq. Quarterly Journal of the Geological Society, vol. xxxi. No. 124, p. 519.

most of the valuable deposits of coal in India, and this coal has sometimes been regarded as palæozoic, either Permian or Carboniferous, chiefly on the ground of the supposed resemblance of its flora to that of certain coal-bearing deposits in Australia. Dr. Feistmantel, however, after a careful comparison of the flora of the Indian with that of the Australian coal, concludes that such a correlation is untenable, and that we must rather seek the representatives of the Indian coal plants in the Triassic beds of the continent of Europe. If then the evidence of fossil botany is to decide the question, we must admit that most of the Indian coal was formed about the time when the New Red Sandstone was being deposited in this country.

Yet as a

When a shower of rain falls upon the ground, it dissolves more or less of the soluble constituents of the soil, and carries them sooner or later to the river, whence they are ultimately borne out to sea. Held invisibly in solution, these dissolved impurities are apt to escape notice, and have consequently received from geologists less attention than has been bestowed upon the solid impurities which are visible by the turbidity which they impart to the water in which they are mechanically suspended. Whilst therefore we have had many estimates of the quantity of sedimentary matter abraded from the land and carried to the sea, but little has been done towards determining the amount of mineral matter removed in a state of chemical solution. geological agent the one is as worthy of study as the other. The question has therefore been recently attacked by Mr. Mellard Reade, who has based his calculations mainly on Dr. Frankland's elaborate analyses of river waters, published in the Sixth Report of the Rivers' Pollution Commission.

Mr. Reade's first problem is to estimate the total quantity of solid material removed in the course of a year, by the solvent action of rain, from the entire surface of England and Wales. For this purpose he takes the mean rainfall of the country as thirty-two inches. It is notable that the variation of rainfall in different parts is not found to affect the aggregate quantity of dissolved matter to anything like the extent that might have been anticipated. True, the hilly districts of the west, in Cumberland, Wales, Cornwall, and Devon, intercept a large quantity of rain; but it must be remembered that these collecting-grounds are composed of old rocks, ranging from the Cambrian to the Carboniferous, and that such rocks are to a great extent insoluble, so that the rivers which drain them are comparatively pure. On the other hand, in the southern and eastern counties, as in the Thames basin, the rainfall is much less than in the west ; but then the rocks generally belong to Secondary or Tertiary formations, and are tolerably soft and soluble. A kind of compensation is

On Geological Time. Presidential Address to the Liverpool Geological Society. By T. Mellard Reade. Liverpool, 1877.

« AnteriorContinua »