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Nature Series. Polarization of Light. By WILLIAM SPOTISWOODE,
M.A., LL.D., F.R.S., 8c 'London: Macmillan and Co. 1874
(crown 8vo, pp. 129). THIS work contains an admirably clear and exact account of the
phenomena of Polarized Light. The author explains, in the first place, the various ways in which light is made to undergo the process of plane polarization (by passage through a doubly-refracting crystal and by reflexion and refraction by glass), and then describes the double-image prism, Nicol's prism, and Norremberg's apparatus. The subjects of the interference of plane-polarized light, the methods of producing circularly polarized light, and the interference of circularly polarized light are next discussed at considerable length with reference to pencils of parallel rays. These subjects fill two thirds of the volume; but, as well as the main subject, several collateral points of great interest come under notice in the course of the discussion: such are the various forms of the saccharometer, Sir C. Wheatstone's apparatus for producing circularly polarized light by reflexion at a metallic surface, and exhibiting the effects of passing light in that state through crystal films, the phenomena resulting from the action on polarized light of glass when unequally cooled, or when in a state of stress, the phenomenon of atmospheric polarization, the polar clock, &c. The student who has made out this part of the volume will be able to understand easily the account which follows of the results arising from the use of a pencil of divergent rays, and the beautiful phenomena of coloured rings produced by the action of various crystals on such a pencil. In several places the author notices the results of an examination of the spectrum of the pencil of interfering light which issues from the analyzer ; and a good deal of attention is paid to the production of complementary colours by polarized light. In fact the last chapter of the volume is a reprint of the author's paper “On Combinations of Colour by means of Polarized Light," originally published in the · Proceedings of the Royal Society,' vol. xxii.
It will be seen from the above brief account of its contents that the work before us passes in review all the main points of interest in the subject. The author speaks of his book
talk rather than a treatise on polarized light," and says that it contains “the substance of lectures delivered at various times to my work-people." It is therefore so far a popular book, that the subject is discussed without the aid of mathematical symbols ; and doubtless, when the experiments described were actually exhibited, the lectures would be very interesting to any intelligent audience. This, however, does not prevent the book from being decidedly hard reading; and it would be easy to point out passages which could not be fully understood by a reader wholly without the mathematical learning which the author has at command. On the other hand, a reader who already has some acquaintance with the mathematical treatment of the subject will find the time usefully spent which he de
votes to the study of a work like the present, which gives with great clearness, accuracy, and brevity the facts of the subject clothed, as far as may be, in the language of daily life.
The illustrations are sufficiently numerous : some of them are little more than diagrams; others are elaborate drawings of instruments, and are very good, e.g. those on p. 60 and p. 102. There are also two whole pages of coloured figures (which are very beautifully done), showing the rings and brushes produced by polarized light in the case of six crystals, two complementary figures for each crystal. In conclusion we can cordially recommend the book to the somewhat select class of readers who take an intelligent interest in this beautiful branch of science.
IX. Proceedings of Learned Societies.
[Continued from vol. xlviii. p. 546.] February 4, 1874.-His Grace the Duke of Argyll, K.T., F.R.S.,
President, in the Chair.
1. “The Physical History of the Valley of the Rhine." By Prof. A. C. Ramsay, LL.D., V.P.R.S., Vice-President.
The author first described the general physical characters of the valley of the Rhine, and discussed some of the hypotheses which have been put forward to explain them. His own opinion was that during portions of the Miocene epoch the drainage through the great valley between the Schwarzwald and the Vosges ran from the Devonian hills north of Mainz into the area now occupied by the Miocene rocks of Switzerland. Then after the physical disturbances which closed the Miocene epoch in these regions, the direction of the drainage was reversed, so that, after passing through the hill-country between the lake of Constance and Basel, the river flowed along an elevated plain formed of Miocene deposits, the remains of which still exist at the sides of the valley between Basel and Mainz. At the same time the Rhine flowed in a minor valley through the upland country formed of Devonian rocks which now constitute the Taunus, the Hundsruck, and the highland lying towards Bonn; and by the ordinary erosive action of the great river the gorge was gradually formed and deepened to its present level. In proportion as the gorge deepened, the marly fat Miocene strata of the area between Mainz and Basel were also in great part worn away, leaving the existing plain, which presents a deceptive appearance of having once been occupied by a great lake.
2. “ On the Correspondence between some Areas of Apparent Upheaval and the Thickening of Subjacent Beds.” By W. Topley, Esq., F.G.S., Geological Survey of England.
The author referred to many instances in which beds have unequal development, being much thicker in some places than in others; and the main object of his paper was to show that such thickening and thinning of beds has an important effect in producing the apparent dip of overlying beds. The thinning of any one bed may have an appreciable effect in producing or increasing its own apparent dip; but where a whole series of beds thin constantly in one direction, the amount of the dip of one of the higher beds, due to the sum of the thinnings of the underlying beds, is often very considerable.
In illustration of this the author indicated the Lower Jurassic rocks between Leckhampton Hill and Burford. The Inferior Oolite, and the Upper, Middle, and Lower Lias thin out rapidly to the east along this line; the base of the Middle, Lias is nearly a horizontal line, the easterly “ dip" of the Great Oolite being due to the easterly thinning of the Middle and Upper Lias and the Inferior Oolite. The base of the Lower Lias has a westerly dip.
It is generally supposed that the dip of any bed is due to great movements of the earth's crust; from the facts mentioned the author argued that our inferences as to such movements will vary according to the beds which happen to be exposed at the surface. In the example given we assume a westerly upheaval because we see the Great Oolite dipping to the east. If over this area the whole of the higher strata had been removed down to the Middle Lias, we should perceive the beds to be flat; if the denudation had exposed the base of the Lias, we should probably suppose that along this line there had been an easterly upheaval.
Under London the entire series of strata between the Palæozoic rocks and the base of the Upper Cretaceous is absent; as we recede from the London Basin the intermediate beds necessarily come in and gradually thicken, producing the dip of the Cretaceous beds towards the London Basin.
The Palæozoic rocks under London are about 800 feet below sea-level. We now know, by the sub-Wealden boring, that under the centre of the Weald the Palæozoics are also below sea-level. Supposing that they should be found at about the same depth as at Kentish Town, then the Palæozoic floor will be approximately a straight line, whilst we know that the whole of the Wealden and other cretaceous beds dip to the north, their dip being thus wholly accounted for by the gradual thinning out of all the beds as they approach London. In the case of the Weald, some of the strata are of freshwater origin; we must suppose that the area of water within which they were deposited had some limit to the north, and that the Palæozoic floor, which is now approximately flat, rose up to the north as a bounding ridge. But even if this be so, it is evident that the present dips are no exact measure of the amount of upheaval which the beds have undergone.
Other examples were quoted in which the observed general dip corresponds in direction with the known or inferred general thinning; and it was shown that in all such cases we are liable to serious error in inferring the amount of upheaval from observed dips.
It is, however, evident, from the faults intersecting strata, that upheavals and disturbances have taken place; but unless we assume every bed to have been deposited on a perfectly horizontal plane, we cannot infer the amount of such upheaval from the present position of the bed. In all cases we must take into account the actual or possible thinning of underlying beds.
The beds which support Geological Basins frequently thin towards the centres of those basins, thus producing, wholly or in part, the basined form of the strata. It was, however, shown that the beds of the basins themselves frequently thicken towards the centre of the basins. February 25th.—John Evans, Esq., F.R.S., President, in the Chair.
The following communications were read :
1. “ Geological Notes on a Journey from Algiers to the Sahara.” By George Maw, Esq., F.L.S., F.G.S.
The author commences by describing the details observed on his journey from Algiers to L’Aghouat, on the borders of the Sahara. The distance traversed was 285 miles, or about 210 miles in a straight line, and in a direction nearly north and south. No eruptive rocks were observed. The oldest rock is a boss of mica-schist and gneiss behind the city of Algiers; it forms a low anticlinal, with a N. and S. strike. The pass through the gorge of the Chiffa in the Lesser Atlas shows hard slaty rocks dipping S. at a high angle; they are repeated as an anticlinal on the south side of the higher part of the Tell plateau, and are probably Mesozoic. In the plain separating the Tell from the Hauts Plateaux, and on the south side of the latter, red and yellow sandstones form anticlinals; these rocks resemble the Bunter in mineral characters, and are overlain by red marls resembling the Keuper. In the northern escarpment of the Hauts Plateaux saliferous marls are exposed, interstratified between the sandstones and below the red and grey marls. Crystals of salt and gypsum are intimately mixed with the grey marls; and the so-called “ Rochers de Sel” are capped with great blocks of rock tumbled about in confusion, the position of which the author ascribes to the failure of support due to the solution of the salt in the underlying salt-marls. A thin series of bright red and green marls is seen to overlie the Red Sandstones in several places; and above this is an immense series of dark grey marls, interstratified with argillaceo-calcareous bands, forming a great synclinal of the Hauts Plateaux, and a contorted mass on the Tell plateau. These are probably cretaceous. At L’Aghouat they are overlain by fossiliferous beds, probably of Miocene age. Other Tertiary beds observed are soft yellow calcareous freestones on the flanks of the promontory of Algiers and of the Lesser Atlas, and some red and grey marls and ferruginous freestone capping the Tell plateau, the former at a height of 100-900 feet, and the latter of 2500-4000 feet above the sealevel. The plain of the Mitidja, between the Lesser Atlas and Algiers, consists of grey loam with shingle-beds, of post-tertiary age. A similar loam covers the great plain of the northern Sahara, and rises to a height of 2700 feet. Raised beaches occur on the coast up to an elevation of 600 feet above the sea-level; and similar beaches are found inland, south of the Tell plateau, at a height of 2000 feet.
The oldest land in the line of section is the anticlinal of mica schist near Algiers, the strike of which is nearly at right angles to that of the other rocks. The upheaval of the Mesozoic rocks was contemporaneous with the first upheaval of the Lesser Atlas ; it was followed by a long period of denudation, and this by a subsidence of at least 3000 feet in Tertiary times, during which the Miocene deposits were formed. The Tell plateau was thus elevated at least 4000 feet, and the district north of the Lesser Atlas at least 1000 feet, the north face of those mountains probably marking a posttertiary line of fault of 3000 feet. This operation was followed by a long period of denudation, and this by a post-tertiary depression, which the author terms the “ Sahara submergence,” after which the land was reelevated at least 3000 feet, but perhaps considerably
A gradual subsidence appears to be still taking place.
2. “On the Trimerellidæ, a Palæozoic Family of the Palliobranchs or Brachiopoda." By Thomas Davidson, Esq., F.R.S., F.G.S., &c., and Prof. William King, Sc.D.
In this memoir the authors describe in detail certain Brachiopoda, for which they propose to establish a distinct family, discuss the characters and affinities of the family, and indicate certain geological considerations which arise from their study of its members. The first known species were described in 1853 under the names of Obolus Davidsoni and 0. transversus ; but in 1862 Mr. Billings described an allied form as constituting a new genus called Trimerella. With this Gotlandia of Dall (1870) is identical. In 1871 Prof. Hall proposed the new genera Rhymobolus and Dinobolus ; and in the same year Mr. Billings established the genus
Monomerella. Rhynobolus is regarded by the authors as a synonym of Trimerella ; and several other proposed genera being similarly cancelled, the authors retain only the genera Trimerella, Dinobolus, and Monomerella as constituting their family Trimerellidæ.
The Trimerellidæ are described as having a calcareous shell, generally massive, with the umbo of the pedicle-valve often large, pointed, solid or hollow; area usually of considerable size, with a large solid deltidium ; hinge of both valves slightly dentary ; in the interior of each valve there is a more or less elevated platform, longitudinally placed, and either solid or doubly vaulted ; and from the
middle of its anterior end a median plate occasionally projects into the anterior half of the valve, especially in the brachial valve.
In discussing the affinities of the new family, the authors enter in considerable detail into the arrangement and functions of the muscles of the Brachiopoda, and show, from the muscular indications and other characters of the Trimerellidæ, that they are most nearly allied to the Lingulidæ, although presenting characters of sufficient importance to warrant their separation as a distinct family.
As far as is at present known, the Trimerellidæ are confined to the Cambro-Silurian and Silurian systems; but the authors indicate the probability that in the Cambrian or early Cambro-Silurian rocks a generalized form will hereafter be detected, bringing Discina, Lingula, and Obolus into close relationship, as regards their myology, with each other and with Dinobolus. Such a form would constitute the root from which the Trimerellidæ have originated. The
genera and species described are as follows :: Genus 1. Trimerella, Billings (1862). Sp. 1. T. grandis, Bill. ; 2. T. acuminata, Bill. ; 3. T. Lindströmi, Dall; 4. T. Billingsü,