3

the name of cleavage has been given, and which seems to be altogether independent of the stratification; since, even where the strata are curved or variously contorted, the planes of cleavage retain a uniform direction throughout large tracts of country. Mr Bakewell had made some interesting observations on this subject, in the Alps of Switzerland and the Savoy; and Mr Phillips, who has carried the enquiry still further in the north of England, we believe, is still occupied upon it. But it is to Professor Sedgwick that the principal generalization yet made is to be ascribed, which, if confirmed by more extensive enquiry, will probably lead to the knowledge of a new class of facts in the structure of the globe. An example, in his own forcible language, will best illustrate his views:

--:

A rugged country, more than thirty miles in length, and eight or ten miles in breadth, stretching from the gorge of the Wye, above Rhaiada, in North Wales, to the upper gorges of the Elan, exhibits, on a magnificent scale, thousands of examples like that represented below:

·-

The whole region is made up of contorted strata, and of the true bedding there is not the shadow of a doubt. Many parts are of a coarse mechanical structure, but, subordinate to them, are fine crystalline chloritic slates. But the coarser beds and the finer, the twisted and the straight, have all been subjected to one change. Crystalline forces have rearranged whole masses of them, producing a beautiful crystalline cleavage, passing alike through all the strata. And again, through all this region, whatever be the contortions of the rocks, the planes of cleavage pass on, generally without deviation, running in parallel lines from one end to the other, and inclining, at a great angle, to a point only a few degrees west of the magnetic north. Without considering the crystalline flakes along the planes of cleavage, which prove that crystalline action has modified the whole mass, we may affirm that no retreat of parts, no contraction of dimensions, in passing to a solid state, can explain such phenomena as these. They appear to me only resolvable on the supposition that crystalline or polar forces, acted on the whole mass simultaneously, in given directions, and with adequate power. It is not, however, necessary to suppose that these effects were produced in a short lapse of time. In speculating on the time required for the completion of these phenomena, we are free from any unnecessary limitations."*

In the eleventh chapter, some remarkable instances are quoted

*Geological Transactions,' Second Series, Vol. III., p. 477.

VOL. LXIX. NO. CXL.

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from Professor Keilhau,* of the great extent to which metamorphic change is produced in the south-west of Norway, by the proximity of large masses of granite or syenite; the stratified rocks, replete with shells and zoophites, and consisting chiefly of shale and limestone, being there invariably altered near the contact, to distances of from fifty to four hundred yards. The fossils can rarely be detected in these altered schists, and are more completely effaced in proportion to their vicinity to the granite. The limestone, which at remote points is of an earthy texture, blue colour, and often abounds in corals, becomes a white granular marble near the granite, the corals being commonly obliterated, but sometimes preserved even in the white marble. Nothing can more beautifully confirm what Hutton had long ago delivered on this subject, † than these details.

The state of things when great subterraneous masses are exposed to very high temperature, while the volatility of their components is restrained by an enormous pressure, is so remote from any thing which we can imitate or can see, that it is only by effort and long reflection that we can approach to just conceptions with regard to it. It was the sagacious view which Hutton took on this subject, which enabled him to remove the difficulties till then connected with the igneous theory; and in the following passage (which is only one of many equally original) he anticipates some of the speculations since suggested by recent discovery or observation:— If,' he says, 'heat be once admitted as a cause of consolidation, the loose and discontinuous body of a stratum may be closed by means of softness and compression; the porous structure of the materials may be consolidated, in a similar man'ner, by the fusion of their substance; and foreign matter may be introduced into the open structure of strata, in form of 'steam or exhalation, as well as in the fluid state of fusion.' ‡ Here is an evident approach to the hypothesis proposed by Von Buch, which was by many considered as visionary, or as a reference to an occult cause, but is now generally received. That whole mountains of ordinary carbonate of lime had been changed into dolomite, after having become solid, by the pervasion of magnesian vapour, arising from heated trappean masses below, was not so remote from apparent possibility as the silent and unseen transmission of chemical elements by voltaic electricity. Mr Lyell, carrying on this speculation, adduces examples of the effect of gases in causing mineral change.

*Goa Norwegica,' pp. 61-63.

‡ Theory, I., p. 49.

Theory, I., chap. 4.

We know,' he says, that volcanoes in eruption not only emit fluid lava, but give off steam and other heated gases, which rush out in enormous volumes for days, weeks, or years, continuously, and are even disengaged from lava during its consolidation.-(P. 74.) These aeriform fluids,

such as sulphuretted hydrogen, muriatic acid, and carbonic acid, issue in many places from rents in rocks, which they have discoloured and corroded, softening some and hardening others. Now, even the most compact rocks may be regarded, before they have been exposed to the air and dried, in the light of sponges filled with water; and it is conceivable that heated gases brought into contact with them, at great depths, may be absorbed readily, and transfused through their pores.'-Pp. 247, 248.

The Second Part of the volume, in which the Chronology ' of Rocks' is considered, will, to most of our readers, be the most interesting portion, from its involving the relations of strata to their fossil contents, and making us acquainted with innumerable forms and phenomena connected with animal life, of which geology alone produces examples.

Chapter XII. On the Different Ages of the Four Great Classes,' is excellent. The author's arrangement is nearly the same with that of Dr Boué, published in 1825, on which also the subsequent arrangement of Brongniart, of 1829, was founded. The aqueous, plutonic, volcanic, and metamorphic rocks, form four parallel columns, in one chronological series;--and they are to be regarded as four sets of monuments, relating to four contemporaneous, or nearly contemporaneous, series of events.' Chapter XIII. Ages of the Aqueous Rocks.'-The tests of relative age, and the means of identifying contemporaneous portions of the same groups, in the aqueous class of rocks, are three, -superposition, mineral character, and fossils: the uppermost in a series of horizontal strata being of course the newest, and the lowest the most ancient

So that a series of sedimentary formations are like volumes of history, in which each writer has recorded the annals of his own times, and then laid down the book-with the last written page uppermost-upon the volume in which the events of the era immediately preceding were communicated. In this manner, a lofty pile of chronicles is at length accumulated; and they are so arranged as to indicate, by their position alone, the order in which the events recorded in them have occurred.'- P. 212.

The adoption of this principle of superposition, simple as it may appear, is one of the great improvements, if not the chief characteristic, of modern geology; and much of the astonishing progress of the subject, during the last half century, must be ascribed to its steady application.

With respect to mineral character--the second test of age—

it is observed that strata, though sometimes continuously the same even for hundreds of miles, more commonly either thin out or change within much narrower limits. The most definite proof of the identity of strata is derived from their fossil contents* -inasmuch as organic forms are more accurately distinguishable than the shades of mineral character-yet even the former test must be used with much the same restriction. The test by fossils is different from that by mineral composition in this also -that, while the same fossils prevail throughout very considerable spaces in a horizontal direction, we seldom meet with the same remains vertically, or in a line transverse to the stratification, for many fathoms together, although the composition of the beds mineralogically may be very little changed—

This fact has now been verified in almost all parts of the globe, and has led to a conviction, that, at successive periods of the past, the same area of land and water has been inhabited by species of animals and plants as distinct as those which now people the antipodes, or which now co-exist in the arctic, temperate, and tropical zones. It appears that, from the remotest periods, there has been ever a coming in of new organic forms, and an extinction of those which pre-existed on the earth; some species having endured for a longer, others for a shorter time, but none having ever reappeared after once dying out. The law which has governed the creation and extinction of species, seems to be expressed in the verse of the poet, Natura il fece, e poi ruppe la stampa.'-Ariosto.—Nature made it, and then broke the die. And this circumstance it is which confers on fossils their highest value as chronological tests, giving to each of them, in the eyes of the geologist, that authority which belongs to contemporary medals-[as Kirwan, we believe, has called them]-in history.'—Pp. 275, 276.

A list of the groups of strata hitherto observed in Western Europe, is here given; and it is not a little remarkable, that almost all of them are represented in the British Islands. The list is followed by an account (which occupies the remainder of the book) of each principal group in succession—describing its mineral characters, and enumerating its principal fossils; of many of which woodcuts of singular excellence are given, with remarks on their geographical distribution and local situation.

* Our readers are aware that it is to Mr William Smith we are indebted, in England, for the practical knowledge of this great principle and instrument in the distinction of stratified rocks. They will find a statement of the doctrine really advanced and acted upon by Mr Smith, in investigating the relations of the oolitic series, by which his subsequent enquiries were suggested, in the Treatise,' by Professor Phillips, republished from the Encyclopædia Britannica.' -(Edinburgh, 1838: 12mo. Pp. 136-139,) to which we wish especially to refer.

Besides the previously published matter here collected, this portion of the volume contains the result of much original observation, and several facts which to us were new. But we must confine ourselves to a few of the most prominent points in each division.

Tertiary Formations, p. 282 to 291.-The necessity of dividing the tertiary deposits being universally allowed, the principal questions connected with their classification appear to be three; on all of which, it is possible, some uncertainty still remains:~ 1. On what principle shall the division be made? 2. What shall be the number of the divisions? 3. By what names shall they be distinguished?

The basis of the arrangement proposed by Mr Lyell and M. Deshayes, about 1829, was the relative proportion or per centage of fossil shells in the respective groups, in identical species with those now living in existing seas. In the application of this principle, Mr Lyell divided the whole of the European tertiary deposits into four groups, under the names of Eocene, Miocene, and Pliocene, (the last comprising two subdivisions.) These terms were intended to express the relative newness of the groups; the limit of the Pliocene division being, of course, the existing condition of our seas. M. Deshayes, on the other hand, proposed to divide the whole series into three.*

After a time, however, objections to the per centage principle were brought forward by Mr Charlesworth and M. Desnoyers,+ on grounds which are nearly the same. They have clearly shown that, in the application of the principle, the most incongruous results would be produced by discordance between different conchologists, on the question of identity in the species of shells.

A summary of his results has been more recently given by M. Deshayes himself, in the following terms: I. The tertiary strata of Europe contain no species which can be identified with the secondary strata lying beneath them. 2. The tertiary strata are the only ones which contain fossil specimens of existing species. 3. The fossil shells which can be identified with living species, are more numerous in proportion C as the strata are more recent, and vice versa. 4. Constant proportions (3 per cent, 19 per cent, 52 per cent) in the number of recent species, determine the age of the tertiary strata. 5. The tertiary strata are superposed one upon another, and not parallel, as was at first imagined. 6. The tertiary strata, according to their zoology, ought to be divided * into three groups or stages.-[See a paper read before the French Academy, May 1836. Translated in Loudon's Mag. of Nat. Hist.,' Vol. I., N.S., p. 9.)

+Edin. and Lond. Jour. of Science, Vols. VIII. and X., 1836, &c. Bulletin de la Soc. Géologique de France, Avril 1837.