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ays (p. 60) that he "was informed" (which implies that he did not previously know) that "one of Van der Waals' papers contains an elaborate study of the molecular pressure in fluids. Again he says, "I have left the passages. which refer to this ubject in the form in which they stood before I became acquainted with Van der Waals' work. I have not sufficiently | studied his memoir to be able as yet to form a definite opinion whether the difficulty... which is raised in Appendix E. can, or cannot, be satisfactorily met by Van der Waals' methods." Further, he states that he "had been under the impression that Laplace's views had gone entirely out of fashionhaving made, perhaps, their final appearance . . . about 1850." As a matter of fact, Van der Waals adopted Laplace's views in 1873, and his formula differs from the expression pv RT, only by the introduction of two terms, one of which is obviously in additional pressure such as is deduced from Laplace's theory. I do not think that any reader could be expected to conclude from these passages in Prof. Tait's Addendum that when writing the paper he had long known the "main features of Van der Wals' investigation." To me they seemed to mean that he had not previously been acquainted with Van der Waals' work, nor with his methods, nor with the facts that he studied molecular pressure and adopted Laplace's ideas.

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While, therefore, I willingly submit to Prof. Tait's correction of the phrase that he had never heard of Van der Waals," I cannot admit that, on the evidence then before me, I did him any substantial injustice.

12 I very much doubt whether the distinction between the ultimate volume and the molecular volume can be maintained if the equations are treated as empirical; and even if they are not, I dicabt whether the ultimate volume, as defined by Prof. Tait, has any real physical meaning. The value of when p =∞ is independent of the temperature, whether deduced from the theoretical mula to which Prof. Tait refers (p. 48), or from those of Van er Waals or Clausius: hence it must (from this point of view) le the molecular volume. In the case of Prof. Tait's new elation, which was published after his Report was completed,

which is the only one I had not seen when I wrote the review, the results when we put p = ∞ or T: o, are such as to how that its application to these extreme cases is not legitimate. My own view is that such algebraical solutions are worth very itle, and I only discuss them because I wish to show that if we aimit them at all they justify my treating Prof. Tait's number as 1 estimate of the molecular volume.

3) I cannot say that I think that Prof. Tait's reason is dequate. The Royal Naval College at Greenwich has done bore for our naval officers than he would have us believe, and, dit were not so, the Challenger Reports are not addressed to Lembers of any one profession, nor intended for English-speakng scientific men alone. Their cosmopolitan character is shown by the fact that bound up in the same volume with Prof. Tait's Report is another by a distinguished Belgian geologist.

Foreigners have helped to describe the specimens which our Expedition collected; they will read the Reports which our experts Lave written. It would have required but a few minutes' work, ad a few additional lines of print, to have given the final results in terms which they would have understood at a glance. (4) The analogy is fallacious. Prof. Tait has devised a formula into which he introduces two quantities (age and speed), which are commonly expressed with reference to different units

of time.

I pointed out that he had expressed in the same formula (conPary to common usage) the same quantity (pressure) in terms of two different units, of which one is not ordinarily used by many of those who will make use of his work.

As to the last paragraph, I have only two remarks to make, First, that I think Prof. Tait does himself injustice in rearding a description of apparatus devised by another, and the scovery of a blunder of the Bureau International, as two of he most important things in his Report. Secondly, that 1 nk the imputation of motives should be banished from cientific discussions.

In conclusion, I wish to add that probably I should have left Prof. Tait's defence unanswered if he had not accused me of fairness. I have no desire for any controversy, and no wish to impugn his knowledge of the theory of gases. But he will give my reminding him of the old saying, "Noblesse oblige." A classical research should not be published in a state which lead the reader to the conclusion that the author was only just ecoming acquainted with facts which bear upon his work and have been long before the world. As a reviewer, I formed the

opinion that the Report under discussion was open to this criticism. As a reviewer, it was my duty to express my opinion in all honesty, and, as I hope, in all courtesy.

ARTHUR W. RÜCKER.

Visualized Images produced by Music.

IN the annexed paper, and in her own words, are related the very curious effects produced on a lady friend by certain musical tones and orchestral combinations. They are so very singular, so entirely outside my experience, and, withal, so inexplicable, that I shall be glad if you will give them a place in your columns, in the hope that some of your readers-physiological psychological-may be able to throw some light on them. I should state that the lady is in perfect health, is very intelligent, an accomplished musician, and not at all, in this or any sense, the victim of a disordered imagination. She is quite conscious that these spectral images have only a subjective existence, though visually they have all the vividness of presentment which belongs to realities.

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At the first blush it would seem as though these apparitions were in some way a response to stimuli sent through the auditory nerve; but this, if any, is an imperfcct explanation, since it will be noticed that occasionally these visualized pictures slightly precede the instrument they belong to.

This fact suggests that a state of unconscious expectancy may be a factor in their reproduction, but it fails entirely, I think, to account for their initial appearance. GEO. E. NEWTON.

25 Woodland Road, Gipsy Hill, S.E.

"The sound of an oboe brings before me a white pyramid or obelisk, running into a sharp point; the point becoming more acute if the note is acute, blunter if it is grave. The obelisk appears to be sharply defined and solid if the note is loud, and vague and vaporous if it is faint. All the notes of the 'cello, the high notes of the bassoon, trumpet, and trombone, and the low notes of the clarionet and viola, make me see a flat undulating ribbon of strong white fibres.

"The tone of the horn brings before me a succession of white circles of regularly gradated sizes, overlapping one another. These circles and the ribbon float past me horizontally, but the point of the obelisk seems to come at me.

"In an orchestra, when the violins strike up, after the wind band has been prominent for a time, I see often, but not always, a shower of bright white dust or sand, very crisp and glittering. I am taking note of the recurrence of this impression, and think it is becoming more frequent, but it is not invariable like the others.

"I have heard a great deal of orchestral music all my life, but I have only noticed these effects for four or five years. They gained gradually in frequency and clearness, and now the first three are invariable.

"If I know the scoring of a piece well, the various effects slightly precede the instrument they belong to; only the objects are vague and faint till the sound begins.

46

Sometimes, if an oboe passage has an intense and yearning character, the white point comes so near me, and moves so rapidly, that I think it must wound me.

"I am very anxious to make it clear that I am not trying to describe a mental state by symbols, but that I actually see the point, the fibres, and the circles. Generally they seem to float half-way between me and the orchestra.

"If only one class of instruments is used, the effect does not extend beyond the opening bars: for instance, in a string quartette I only see the white sand for a moment at the beginning; if, however, wind and stringed instruments are combined, I see the various effects again and again in one piece."

Foreign Substances attached to Crabs.

IN your issue of December 26, 1889 (p. 176), Mr. Pascoe drew attention to the cases of certain crabs which are frequently found covered with sponges, algæ, shells, &c., and brought forward also the well-known case of the Gastropod Phorus. He at the same time confessed that he could not see "where protection came in" in any of the cases which he cited. Mr. A. O. Walker, on the other hand (NATURE, January 30, p. 296), regards it as obvious that the attachment of these foreign substances is a useful adaptation for purposes of concealment. Prof. Herdman also (NATURE, February 13, p. 344) bears witness to the

"scarcely recognizable" appearance of the crab Hyas when covered with algæ, &c. Indeed, no one who has seen one of these crabs brought up with the dredge, or has found a well-covered Stenorhynchus on our own shores, can seriously doubt the usefulness of the habit in rendering the animal inconspicuous. In Stenorhynchus and Inachus the process of "dressing" with weeds and zoophytes has been described by Bateson (Journ. Mar. Biol. Association, vol. i. 1889, p. 213), and it is seen from his description that, as also in the cases of Dorippe, Pagurus, Dromia vulgaris, &c., the foreign substances or animals become attached to the body not by accident but by the act of the crabs themselves.

Now Mr. Walker, in regarding all these cases as instances of adaptation for concealment, has overlooked the fact that in two of our British species of hermit crab (Pagurus bernhardus and P. prideauxii) it is the habit of the animals to prefer, and often to fight for, shells which are rendered conspicuous by the attachment to them of species of Anemone, in the one case Adamsia rondeletii (Sagartia parasitica), in the other Adamsia palliata. Another British species (Pagurus cuanensis) is almost invariably found inhabiting a shell enveloped in the sponge Suberites domuncula, which is frequently of a conspicuous orange-red colour Only in the smallest species of Pagurus (e.g., P. lævis) does the animal depend invariably upon an inconspicuous appearance for its safety.

The value to the crabs of a preference for shells to which Actinians are attached is found in the fact that these gailycoloured animals are carefully shunned by fishes on account of their stinging powers; and although hermit crabs themselves are very palatable to fishes, their association with Actinians, while rendering them conspicuous as they move about, is at the same time an efficient protection from the persecution of their enemies.

This also explains the habits of the two Mauritian crabs, which, according to Möbius, carry about a sea anemone in each claw.

The sponge with which Pagurus cuanensis is associated is (like all other sponges with which I have experimented) exceedingly obnoxious to fishes on account of its bad smell and taste.

I

have never succeeded in inducing a fish of any species to swallow a fragment of the sponge; but on the contrary the smell is in most cases quite sufficient to drive the fish away. The association with the sponge is therefore here also an efficient protection, for I know of no fish capable of extracting the crab from its

retreat. It is seen from this that the case of Dromia vulgaris

should probably be removed from the category of adaptations for concealment, and, like the cases of P. bernhardus, &c., be included in a special group of warning adaptations.

There yet remains the interesting case, adduced by Dr. R. von Lendenfeld, of Dromia excavata associated with a Compound Ascidian of the genus Atopogaster (Herdman). This, I believe, will be found to belong to the same category of warning adapta tions, for after repeated experiments with Compound and other Tunicata at the Plymouth Laboratory I can state that these animals are essentially inedible to fishes. The inedibility is in large part due, as in the case of sponges, to the characteristic odour which Tunicata, and more especially Compound Tunicata, give out, and in no family (excepting perhaps the Botryllida) is this better marked than in the Polyclinida, the group to which Atopogaster belongs. Bearing in mind also the fact that Composite Ascidians frequently vie with sponges and Actinians in the possession of varied and conspicuous colours, it is rendered practically certain that the case of Dromia excavata is another instance of this same type of adventitious warning contrivances. Thus the edible (the edibility is not yet proved for foreign species) Crus acea which attach foreign substances to their bodies may be divided into two groups :

(a) Those which are rendered inconspicuous in relation to their natural surroundings by the habit; e.g., Stenorhynchus, Hyas, Dorippe, Pagurus lævis, and young forms of Pagurus bernhardus, &c.

(B) Those which associate themselves with animals, easily recognizable by, and possessing qualities offensive to, their chief enemies; e.g., Dromia vulgaris and excavata, Pagurus bernhardus, prideauxii, and cuanensis. WALTER GARSTANG. Laboratory of the Marine Biological Association, Plymouth, February 21.

P.S. From facts which Mr. Weldon and Mr. Harmer have

communicated to me, it would appear that Dromia vulgaris frequently attaches Compound Ascidians (Leptoclinum maculosum,

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THE DISCOVERY OF COAL NEAR DOVER

THE question of the existence of coal under the newe rocks of Southern England, which has engaged the attention of some of our leading geologists since the year 1855, has found its final answer in the discovery announced last week in the daily press. The story of the discovery is a striking example of the progress of a scientific idea, passing through various phases, and growing more clearl defined through opposition and failure, until ultimately has been proved to be true, and likely to lead to industral changes of national importance.

The question was originally started 35 years ago b Mr. Godwin-Austen in a memorable paper brought before the Geological Society of London, in which it was arged, from the character and arrangement of the coal-fields and associated rocks of Somersetshire and South Wales on the west, and of the Belgian and North French coal-fields on the east, that similar coal-fields lie buried beneath the newer strata of the intervening regions. Mr. GodwinAusten pointed out that the general direction of the exposed coal-fields was ruled by a series of great eas. and west folds, running parallel to the great line of dis turbance-"the axis of Artois,"-from the south of Ireland, through South Wales and Northern Somerset on the west, eastwards through Belgium and Northern France. into the valley of the Rhine, near Dusseldorf. Through out this area the exposed coal-fields lie in long east and west troughs. This series of folded Carboniferous and older rocks formed also an east and west ridge along the line of the axis of Artois, which gradually sank beneath the waves of the Triassic, Liassic, Oolitic, and Cretaceous seas. Against this the strata of the three first of these rocks gradually thin off, while the coal-measures and other rocks of the ridge have repeatedly been struck in France and Belgium, and are now being worked immediately underneath the Cretaceous strata over a wide

area.

The axis of Artois also, where it is concealed by the newer rocks in the south of England, is marked from Somerset eastwards by the anticlinal of the chalk of North Wiltshire, and the line of the North Downs, the general law seeming to be "that when any great folding and dislocation of the earth's crust has taken place, each subsequent disturbance follows the very same lines, and that simply because they are lines of least resistance."

Mr. Godwin-Austen, by combining all these observa tions, finally concluded that there were coal-fields beneath the Oolitic and Cretaceous rocks of the south of England.

and that they were sufficiently near the surface to allow of their being of great economic value. He further specified the line of the Thames Valley, and the region of the Weald, as possible places where they might be discovered.

These important conclusions were during the next II years generally received by geologists, with the exception of Sir Roderick Murchison. The next important step in the direction of their verification was that taken by the Coal Commission of 1866-67, by whom Mr. GodwinAusten and Sir R. Murchison were examined at length, and the results of the inquiry embodied in the Report by Mr. Prestwich. In the Report, Mr. Godwin-Austen's views are accepted, and fortified by a vast number of details relating both to the coal-fields of Somersetshire and of France and Belgium. Mr. Prestwich also calls special attention to the physical identity of the coals of these two regions, and to the fact that the Carboniferous and older rocks in both are similarly disturbed. He concludes, further, that the coal-fields which now lie buried beneath the newer rocks are probably equal in value and in extent to those which are exposed in Somerset and South Wales on the west, and in Belgium and France on

the east.

It

In 1872 the Coal Commission Report was published, and in the same year the Sub-Wealden Exploration Committee was organized by Mr. Henry Willett to test the question of the existence of coal in the Wealden area by an experimental boring. The site chosen was Netherfeld, near Battle, in Sussex, where the lowest rocks of the Wealden formation form the bottom of the valley. was resolved to go down to the older Palæozoic strata, which were thought to occur at about 1000 feet from the surface, or to carry the bore-hole to 2000 feet if they were not struck before. The work was carried on under considerable difficulties for the next three years, until in 1875 it had to be abandoned at a depth of 1905 feet, because of the breakage of many hundred feet of lining-pipes, Coupled with the loss of the boring-tool at the bottom. The section of the strata passed through is as follows:

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of Wealden rocks were present, they were more than 1000 feet thick.

For the next eleven years the problem remained where it was left by the results of the Netherfield boring; while in the district of London, evidence was being collected in various sinkings for water, which proved the existence of the Paleozoic ridge of rocks, Silurian and old red sandstones, older than the Carboniferous, at about 1000 feet from the surface. Here, too, the Oolitic strata were not more than 87 feet in thickness, at their thickest point in the well at Richmond. The older rocks, moreover, were inclined at a very high angle, as in the case of the similar rocks underlying the coal-fields of Somerset, and of Northern France and Belgium, and this implied the existence of troughs of coal-measures in the synclinal folds, in neighbouring areas.

I come now to the last experiment, which has been so fortunately crowned with success. In 1886, I reported to Sir Edward Watkin that it was desirable, both on scientific and commercial grounds, for a boring to be put down in South-East Kent, in the neighbourhood of Dover, and that the Channel Tunnel works under the Shakespear Cliff would be the best site for the experiment. It was almost within sight of Calais, where the coal-measures had been proved at a depth of 1092 feet. It was also not many miles away from the spot where a large mass of bituminous material-which, according to Mr. Godwin-Austen, was the result of the distillation of coal from the measures beneath-had been discovered in the chalk. Sir Edward Watkin acted with his usual energy on my report, and the work was begun in 1886, and carried on, under my advice, down to the present time. The boring operations have been under the direction of Mr. F. Brady, the chief engineer of the South-Eastern Railway, to whose ability we owe the completion of the work to its present point, under circumstances of great difficulty. The strata passed through may be generalized as follows:

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This section, although it yielded no information as to the Palæozoic rocks, showed that in this particular district they are more than 1900 feet beneath the surface, and revealed the great thickness of the Kimmeridge Clay and Corallian rocks, sufficiently distant from the ridge of coal-measures and older rocks, against which the Oolitic strata thin away to the north, to allow of an accumulation of Oolitic sediments to a thickness of more than 1700 feet. In this respect, therefore, it afforded unmistakable evidence that the search for the ridge in question might be carried on with much greater chance of success further to the north, in the direction of the North Downs. The great and increasing thickness of the successive newer rocks of the Wealden formation, which form the surface of the ground between Netherfield and the North Downs, rendered it undesirable to repeat the experiment within the Wealden area proper. Close to Battle, the Secondary strata were of great thickness, and where the whole series The Committee consisted of Profs. Ramsay and Phillips, Sir John Lubk, Sir Philip Egerton, and Messrs. Thomas Hawksley, Warington th, Prestwich, Bristow, Etheridge, Boyd Dawkins, and Topley. The precive boundary between these two groups is uncertain. If the Kmendge Clay senes be taken down to the Coralline Oolite, its thickness wilbe 151a fect.

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The coal-measures were struck at a depth of 1160 feet, or 68 feet below the point where the coal-measures were met with in the boring at Calais. It may also be noted as a remarkable confirmation of Mr. Godwin-Austen's views as to the abrupt thinning off of the Wealden strata, that, although along the line of the North Downs the Weald clay strikes towards the French coast, and is seen at low water between Hythe and Folkestone, it and the underlying Wealden strata are not represented in the section at the Shakespear Cliff.

It is too soon as yet to measure the full value of this discovery near Dover, while our work is as yet unfinished. We may, however, remark that the coal-fields of the Continent, which have been proved beneath the newer rocks in Northern France and Belgium, some 60 miles to the west of their eastern outcrops, have now been traced across the Channel, that they are at a workable depth, and that we have now a well-defined base for further researches in Southern England.

W. BOYD DAWKINS.

THE RELATION BETWEEN THE ATOMIC VOLUMES OF ELEMENTS PRESENT IN IRON AND THEIR INFLUENCE ON ITS MOLECULAR STRUCTURE.

IN a lecture on the Hardening and Tempering of Steel,

published in November last (NATURE, vol. xli. pp. 11, 32), an attempt was made to set forth the prominent facts developed in recent researches, more especially those of M. Osmond, which tend to prove that iron, like many other elements, can pass from the normal state to an allotropic one. It was shown that as a mass of iron or steel cools down, there are at least two distinct evolutions

of heat, one occurring at a variable temperature not higher than 855 C., the other at a more constant temperature, near 650° C. From a long series of most patient investigations, Osmond argues that there are two kinds of iron, one [hard] Biron, and the other [soft] a iron. The molecular change from ẞ to a iron is indicated by the first evolution of heat in the cooling mass of iron or steel, and at this point the cooling mass of iron regains the magnetic properties which it loses at higher temperatures. The second evolution of heat only occurs in carburized iron or steel, and marks the point at which carbon itself changes from the dissolved or hardening-carbon,' to the state of combined or 'carbide-carbon.' In highly carburized steel, the two points at which heat is evolved coincide, and experimental evidence has been given (loc. cit. p. 34) as to the abnormal molecular weakness which is exhibited when a very hot bar of such steel cools down to about 660° C. In a recent communication to NATURE (February 20, p. 369), Prof. Carl Barus, of Washington, has pointed out, with reference to this molecular weakness, "that when iron passes through the temperature of recalescence its molecular condition is almost chaotic"; whilst with regard to Osmond's view that a iron passes to B iron when submitted to any stress which produces permanent deformation of the mass, Prof. Barus says that "there is reason to be urged even in favour of the extreme view" that such molecular change may be produced in most metals. In the lecture at Newcastle, l'expressed the belief (NATURE, loc. cit.) that it would be shown that the influence of small quantities of other elements on masses of iron would be found not to be at variance with the periodic law. I had already given experimental evidence to show that the action of small quantities of impurity on the tenacity of gold was closely in accordance with that law, but in the case of iron it was difficult to say what property of the metal would be most affected by the added matter. It appeared safe, however, to point to the possibility that the direct connection with the periodic law would "be traced by the effect of a given element in retarding or promoting the passage of ordinary iron to the allotropic state," a point of much importance, as the mechanical properties of the metal must depend on the atomic arrangement in the molecules.

I am glad that so eminent an authority and admirable experimenter as M. Osmond has satisfied himself as to the probable accuracy of this view. In two recent papers communicated to the Académie des Sciences, the results of his experiments are given, and the following is a translation of the later of these (Comptes rendus, vol. cx. p. 346):

"Within the last few years and quite recently (Comptes rendus, Séances des 26 octobre et 6 décembre 1886, 4 avril 1887, et 3 février 1890), I have had the honour to submit to the Academy facts relating to the allotropic modifications of iron, and to the part played in such changes by foreign bodies alloyed with the mass. Prof. Roberts-Austen, by studying the effect produced on the mechanical properties of gold by the addition of the same weight (about o 2 per cent.) of seventeen foreign metals, has discovered a curious relation between the results ob

tained and the position occupied by the added merals an the periodic classification (Phil. Trans. Roy. Soc., vol clxxix. p. 339, 1888). Prof Roberts-Austen has dedured from this that an analogous relation should exist for iron but the irons and steels of commerce are such complex

products, and the same metal may assume such differen

aspects, that the relation in question is not readily apparen from a study of their mechanical properties.

"In reviewing my former experiments with these ne. ideas as guides, it appeared to me that the law of Robens Austen was well based, and new experiments undertake: to verify it have only confirmed my first view. of iron I have studied experimentally with more or less "The foreign elements whose action on the critical points completeness, are ranged as follows in two columns in the

order of their atomic volumes :

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"The elements in column I., whose atomic volumes re smaller than that of iron (72), delay during cooling cæteris paribus, the change of 3 [hard] iron to a [soft] aron, as well as that of 'hardening-carbon' (carbone de tremp into 'carbide-carbon' (carbone de recuit). For the two reasons they tend to increase, with equal rates of cooling, the proportion of B iron that is present in the cooled iron or steel, and consequently the hardness of the metal. Indeed, their presence is equivalent to a more or less energetic hardening.1

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On the other hand, the elements of column II, whose atomic volumes are greater than that of iron, tend t raise or at least to maintain near its normal position during cooling, the temperature at which the change of to a iron takes place; further, they render the inverse change during heating more or less incomplete, and usually hasten the change of hardening-carbon' to carbide-carbon.'"

"Thus they maintain the iron in the a [soft] state at bh temperatures, and must therefore have the same effect in the cooled metal. In this way they would act on iron as annealing does, rendering it soft and malleable, did not their individual properties, or those of their compound: often intervene and partially mask this natural conse quence of their presence.

"The essential part, therefore, played by foreign elemen.alloyed with iron, is either to hasten or delay the passa of iron, during cooling, to an allotropic state, and m render the change more or less incomplete in one sense or the other, according to whether the atomic volume of the added impurity is greater or less than that of iro In other words, foreign elements of low atomic volume tend to make iron itself assume or retain the particular molecular form that posses es the lowest atomic volume. whilst elements with large atomic volume produce the

inverse effect.

"It should be noted that carbon, whilst obeying the general law, possesses on its own account the property of undergoing, at a certain critical temperature, a change the nature of which is still disputable, although its existence s acknowledged. It is this property which gives carbon a place by itself in the metallurgy of iron."

M. Osmond has shown me the curves which represent the results of his experiments, and these will doubtles To the elements of column I. hydrogen may be added As in known, this element renders electro-deposited iron hand and brittle, pe it would be better to say with Graham Aydrogenism, for hydrogen ga not appear to have a marked influence on the critical temperature. 2 Tungsten alune presents certain anomalies.

soon be published. Whatever may ultimately prove to be the true nature of the molecular change which accompanies the thermal treatment of iron and determines its mechanical properties, there is little doubt but that there is a close relation between the action of foreign elements and their atomic volume. Few metallurgial questions are of greater interest at the present time than those which relate to the molecular structure of metals, and the admirable work of M. Osmond has shown it to be very probable that the presence of a small quantity of a foreign metal may cause a mass of another metal to pass into an allotropic state. In relation to iron and steel the problems are of great industrial importance, and it is fortunate that we appear to be nearing the discovery of a law in accordance with which all metallic masses are influenced by "traces." W. C. ROBERTS-AUSTEN.

SEDGWICK AND MURCHISON: CAMBRIAN AND SILURIAN

ERRONEOUS impressions have long existed among American geologists with regard to the relations to one another, and to Cambrian and Silurian geology, of Sedgwick and Murchison. The Taconic controversy in this country served, most unreasonably, to intensify feelngs respecting these British fellow-workers in geology, and draw out harsh judgments. Now that right views on the American question have been reached, it is desirable that the facts connected with the British question should be understood and justly appreciated.

Sedgwick and Murchison were literally fellow-workers in their earlier investigations. Prof. John Phillips, in a biographical sketch of Sedgwick (NATURE, vol. vii. p. 257), whose intimate friendship through fifty years "he had the happiness of enjoying," speaks thus, in 1873, of their joint work:

"Communications on Arran and the north of Scotland, including Caithness (1828) and the Moray Firth; others on Gosau and the Eastern Alps (1829-31); and still later, in 1837, a great memoir on the Paleozoic strata of Devonshire and Cornwall, and another on the coeval rocks of Belgium and North Germany, show the labours of these intimate friends in the happiest way-the broad generalizations in which the Cambridge professor delighted, well supported by the indefatigable industry of his zealous companion."

Prof. Phillips then speaks of the Cambrian and Silurian labours "of two of the most truly attached and mutually helpful cultivators of geological science in England."

Of these Cambrian and Silurian labours it is my purpose to give here a brief history derived from the papers they published. They were begun in 1831, without concertSedgwick in Wales, Murchison along the Welsh and English borders.

In September of 1831, the summer's excursions ended, Murchison made his first report at the first meeting of the British Association. It was illustrated by a coloured geological map representing the distribution of the "Transition Kocks," the outlying Old Red Sandstone, and the Carboniferous limestone (Murchison, Report of the British Association, i. 91, 1831).

These "Transition Rocks" (of Werner's system), upturned semi-crystalline schists, slates, and other rocks, passing down into uncrystalline, and regarded as mostly non-fossiliferous, the "agnotozoic" of the first quarter of the century, were the subject of Sedgwick's and Murchi son's investigations-the older of the series, as it turned out, being included in Sedgwick's part. They were Printed from advance sheets kindly supplied by Prof. Dana. article appears in the current number of the American Journal of Science. Murchison says, in the introductory chapter of his Silurian System," "No que fin Great Britain, before his investigations began] was aware of the existence below the Old Red Sandstone of a regular series of deposits Contining peculiar organic remains." **From the days of De Saussure and

The

early resolved into their constituent formations by Murchison, and later as completely by Sedgwick in his more difficult field.1

Already in March and April of 1833, Murchison showed, by his communications to the Geological Society of London, that he had made great progress; for the report says:2-He "separated into distinct formations, by the evidence of fossils and the order of superposition, the upper portion of those vast sedimentary accumulations which had hitherto been known only under the common terms of Transition Rocks and Grauwacke." And these distinct formations" were: (1) the Upper Ludlow rocks; (2) the Wenlock limestone; (3) the Lower Ludlow rocks; (4) Shelley sandstones, "which in Shropshire occupy separate ridges on the south-eastern flanks of the Wrekin and the Caer Caradoc"; (5) the Black Trilobite flagstone whose "prevailing Trilobite is the large Asaphus Buchii, which with the associated species," he observed, "is never seen in any of the overlying groups"; and below these, (6) Red Conglomerate sandstone and slaty schist several thousand feet in thickness.

By the following January, 1834, Murchison was ready with a further report, in which he described the "four fossiliferous formations" in detail, and displayed, on a folded table arranged in columns, their stratigraphical order, thickness, subdivisions, localities, and "characteristic organic remains." The subdivisions of the rockseries in the memoir are as follows, commencing above: (I.) Ludlow rocks, 2000 feet; (II.) Wenlock and Dudley rocks, 1800 feet; (III.) Horderley and May Hill rocks (afterward named Caradoc), 2500 feet; (IV.) Builth and Llandeilo flags, characterized by Asaphus Buchii, 1200 feet; and, below these, (V.) the Longmynd and Gwastaden rocks, many thousand feet thick, set down as unfossiliferous.

Thus far had Murchison advanced in the development of the Silurian system by the end of his third year. Upper and Lower Silurian strata were comprised in it, but these subdivisions were not yet announced.

During the interval from 1831 to 1834, Sedgwick presented to the British Association in 1832 a verbal communication on the geology of Caernarvonshire, and another brief report of progress in 1833. A few lines for each are all that was published. The difficulties of the region were a reason for slow and cautious work.

In 1834, as first stated in the Journal of the Geological Society for the year 1852, the two geologists took an excursion together over their respective fields. Sedgwick says (Quarterly Journal of the Geological Society, viii. 152, 1852): "I then studied for the first time the Silurian types under the guidance of my fellow-labourer and friend; and I was so struck by the clearness of the natural sections and the perfection of his workmanship, that I received, I might say, with implicit faith everything which he then taught me." And further, "the whole 'Silurian system' was by its author placed above the great undulating slate-rocks of South Wales." The geologists next went together over Sedgwick's region, and Werner, to our own, the belief was impressed on the minds of geologists that the great dislocations to which these ancient rocks had been subjected had entirely dissevered them from the fossiliferous strata with which we were acquainted."

The term "Transition" early appeared in American geological writings. Sixty to seventy-five years ago it was applied by Maclure, Dewey, and Eaton, to the rocks of the Taconic region and their continuation; for these were upturned, apparently unfossiliferous, semi-crystalline to uncrystalline, and extended eastward to a region of gneisses. The study of the rocks was com menced; but in 1842, before careful work for the resolution of them had been done-like that in which Murchison and Sedgwick were engaged-they were, unfortunately, put, as a whole, into a "Taconic system "of assumed prePotsdam age; at the same time "Transition" was shoved west of the Hudson, over rocks that were horizontal, and already resolved. Owing to this forestalling of investigation, and partly also to inherent difficulties, the right determination of the several formations comprised in this Tacnic or "Transition region was very long delayed.

2 Murchison, Proceedings of the Geol. Soc. London, i. 470, 474. 1833, in a paper on the sedimentary deposits of Shropshire and Herefordshire.

3 Murchison, Proc. Geol. Soc., ii. 13, 1834. The subject was also before the British Association; Report for 1934, p. 652.

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