wherever mountains like Etna, Vesuvius, and the Canary Islands are now the site of eruptions, there are signs of accompanying upheaval, by which beds of ashes full of recent marine shells have been uplifted many hundred feet. We need not be surprised, therefore, if we learn from geology that the continents and oceans were not always placed where they now are, although the imagination may well be overpowered when it endeavours to contemplate the quantity of time required for such revolutions.

We shall have gained a great step if we can approximate to the number of millions of years in which the average aqueous denudation going on upon the land would convey seaward a quantity of matter equal to the average volume of our continents, and this might give us a gauge of the minimum of hypogene energy necessary to counteract such levelling power of running water; but to discover a relation between these great agencies and the rate at which species of organic beings vary, is at present wholly beyond the reach of our computation, though perhaps it may not prove eventually to transcend the powers of Man.

CHAPTER VIII.

CHRONOLOGICAL CLASSIFICATION OF ROCKS.

Aqueous, aërial, plutonic, volcanic, and metamorphic rocks considered chronologically-Terms Primary, Secondary, and Tertiary; Palæozoic, Mesozoic, and Cainozoic explained-On the different ages of the aqueous rocks-Three principal tests of relative age: superposition, mineral character, and fossils-Change of mineral character and fossils in the same continuous formation-Proofs that distinct species of animals and plants have lived at successive periods-Distinct provinces of indigenous species-Great extent of single provinces-Similar laws prevailed at successive geological periods-Relative importance of mineral and palæontological characters-Test of age by included fragments-Frequent absence of strata of intervening periods-Tabular views of fossiliferous strata.

Chronology of rocks.— In the first chapter it was stated that the five great classes of rocks, the aqueous, the aërial, the volcanic, the plutonic, and the metamorphic, would each be considered not only in reference to their mineral characters, and mode of origin, but also to their relative age. In regard to the aqueous rocks, we have already seen that they are stratified, that some are calcareous, others argillaceous or siliceous, some made up of sand, others of pebbles; that some contain freshwater, others

marine fossils, and so forth; but the student has still to learn which rocks, exhibiting some or all of these characters, have originated at one period of the earth's history, and which at another.

To determine this point in reference to the fossiliferous formations is more easy than in any other class, and it is therefore the most convenient and natural method to begin by establishing a chronology for these strata, and then to refer as far as possibleto the same divisions, the several groups of plutonic, volcanic, and metamorphic rocks. Such a system of classification is not only recommended by its greater clearness and facility of application, but is also best fitted to strike the imagination by bringing into one view the contemporaneous revolution of the inorganic and organic creations of former times. For the sedimentary formations are most readily distinguished by the different species. of fossil animals and plants which they inclose, and of which one set after another has flourished and then disappeared from the earth in succession.

In the present work, therefore, the five great classes of rocks will form five parallel, or nearly parallel, columns in one chronological table. They will be considered as sets of monuments. relating to contemporaneous, or nearly contemporaneous, series. of events. Just as aqueous and fossiliferous strata are now formed in certain seas or lakes, while in other places volcanic rocks break out at the surface-so, at every era of the past, fossiliferous deposits and superficial igneous rocks were in progress contemporaneously with others of subterranean and plutonic origin, and some sedimentary strata were exposed to heat, pressure and chemical action, and made to assume a crystalline or metamorphic structure.

The early geologists gave to all the crystalline and non-fossiliferous rocks the names of primitive or Primary, under the idea. that they were formed anterior to the appearance of life upon. the earth; while the aqueous or fossiliferous strata were termed Secondary; and alluviums or other superficial deposits, Tertiary. The meaning of these terms has, however, been gradually modified with advancing knowledge, and they are now used to designate three great chronological divisions under which all geological formations can be classed, each of them being characterised by the presence of distinctive groups of organic remains. rather than by any mechanical peculiarities of the strata themselves. If, therefore, we retain the term 'Primary,' it must not be held to designate a set of crystalline rocks some of which are believed by a few geologists to be even of Tertiary age, but it must be applied to all rocks older than the Secondary formations. Some

geologists, to avoid misapprehension, have introduced the term Palæozoic for primary, from waλacov, ‘ancient,' and (wov, 'an organic being,' still retaining the terms secondary and tertiary; Professor Phillips, for the sake of uniformity, has proposed Mesozoic, for secondary, from peoos, middle,' &c. ; and Cainozoic, for tertiary, from Kaivos, 'recent,' &c. ; but the terms primary, secondary, and tertiary have the claim of priority in their favour, and are of corresponding value.

It may perhaps be suggested that some metamorphic strata, and some granites, may be anterior in date to the oldest of the primary fossiliferous rocks. This opinion is doubtless true, and will be discussed in future chapters; but I may here observe, that when we arrange the five classes of rocks in five parallel columns in one table of chronology, it is by no means assumed that these columns are all of equal length; one may begin at an earlier period than the rest, and another may come down to a later point of time, and we may not be yet acquainted with the most ancient of the primary fossiliferous rocks, or with the newest of the hypogene or netherformed.

Certainly changes must have been going on in the hypogene rocks during the whole of the great periods, but it is thought by many geologists that plutonic rocks, and the very schistose metamorphic rocks are never of late geological age.

For reasons already stated, I proceed first to treat of the aqueous or fossiliferous formations, considered in chronological order or in relation to the different periods at which they have been deposited.

Age of strata.-There are three principal tests by which we determine the age of a given set of strata : first, superposition; secondly, mineral character; and, thirdly, organic remains. Some aid can occasionally be derived from a fourth kind of proof, namely, the fact of one deposit including in it fragments of a pre-existing rock, by which the relative ages of the two may, even in the absence of all other evidence, be determined.

Superposition.-The first and principal test of the age of one aqueous deposit, as compared to another, is relative position. It has been already stated, that, where strata are horizontal, the bed which lies uppermost is the newest of the whole, and that which lies at the bottom the most ancient. So, of a series of sedimentary formations, they 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 commemorated. 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.

In regard to the crust of the earth, however, there are some regions where, as the student has already been informed, the beds have been disturbed, and sometimes extensively thrown over and turned upside down. (See pp. 51 to 69.) But an experienced geologist can rarely be deceived by these exceptional cases. When he finds that the strata are fractured, curved, inclined, or vertical, he knows that the original order of superposition must be doubtful, and he then endeavours to find sections in some neighbouring district where the strata are horizontal, or only slightly inclined. Here, the true order of sequence of the entire series of deposits being ascertained, a key is furnished for settling the chronology of those strata where the displacement is extreme.

Mineral character. The same rocks may often be observed to retain for miles, or even hundreds of miles, the same mineral peculiarities, if we follow the planes of stratification, or trace the beds, if they be undisturbed, in a horizontal direction. But if we pursue them vertically, or in any direction transverse to the planes of stratification, this uniformity ceases almost immediately. In that case we can scarcely ever penetrate a stratified mass for a few hundred yards without beholding a succession of extremely dissimilar rocks, some of fine, others of coarse grain, some of mechanical, others of chemical origin; some calcareous, others argillaceous, and others siliceous. These phenomena lead to the conclusion, that rivers, wind, and marine currents have dispersed the same sediment over wide areas at one period, but at successive periods have been charged, in the same region, with very different kinds of matter. The first observers were so astonished at the vast spaces over which they were able to follow the same homogeneous rocks in a horizontal direction, that they came hastily to the opinion that the whole globe had been environed by a succession of distinct aqueous formations, disposed round the nucleus of the planet, like the concentric coats of an onion. But although, in fact, some formations may be continuous over districts as large as half of Europe, or even more, yet most of them either terminate wholly within narrower limits, or soon change their lithological character. Sometimes they thin out gradually, as if the supply of sediment had failed in that direction, or they come abruptly to an end, as if we had arrived at the borders of the ancient sea or lake which served as their receptacle. It no less frequently happens that they vary in mineral aspect and composition, as we pursue them horizontally. For example, we trace a limestone for a hundred

miles, until it becomes more arenaceous, and finally passes into sand, or sandstone. We may then follow this sandstone, already proved by its continuity to be of the same age, throughout another district a hundred miles or more in length.

Organic remains. This character must be used as a test of the age of a formation or of the contemporaneous origin of two deposits in distant places, under very much the same restrictions as the test of mineral composition.

First, the same fossils may be traced over wide regions, if we examine strata in the direction of their planes, although by no means for indefinite distances. Secondly, while the same fossils prevail in a particular set of strata for hundreds of miles in a horizontal direction, we seldom meet with the same remains for many fathoms, and very rarely for several hundred yards, in a vertical direction, or a direction transverse to the strata. 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 even more distinct than 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 have endured for a longer, others for a shorter, time; while none have ever re-appeared after once dying out. The law which has governed the succession of species, whether we adopt or reject the theory of transmutation, seems to be expressed in the verse of the poet,

Natura il fece, e poi ruppe la stampa.-ARIOSTO.
Nature made him, 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 in history.

The same cannot be said of each peculiar variety of rock; for some of these, as red marl and red sandstone, for example, may occur at once at the top, bottom, and middle of the entire sedimentary series, exhibiting in each position so perfect an identity of mineral aspect as to be undistinguishable. Such exact repetitions, however, of the same mixtures of sediment have not often been produced, at distant periods, in precisely the same parts of the globe; and, even where this has happened, we are not in any danger of confounding together the monuments