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inhabitants of a cold climate or inhabitants of a genial one. They further afford the best of all evidence as to the relative ages of formations—the more recent containing the remains of plants and animals nearly akin to those still peopling the earth, while the more ancient contain plants and animals that differ widely from the existing—and this difference increasing with the age of the formation.

Here, then, by means, first, of superposition, second, by mineral composition, and, thirdly, by fossil remains, the geologist can arrive at the relative ages of the rock-formations that constitute the earth's crust, and can arrange them into sections and systems and periods, just as the historian arranges the reigns and dynasties and periods of human history. As the one speaks of ancient, medieval, and modern times, so the other speaks of primary, secondary, and tertiary systems. As the one groups the populations of the world into ancient, medieval, and modern, so the other groups its life into eozoic, palaeozoic, mesozoic, and cainozoic—that is, dawn-life, ancient-life, middle-life, and recent-life. But while the geologist thus reads the history of the earth mainly through its stratified rocks, he at the same time receives important aid from its igneous or unstratified masses. These, as volcanic products, break through the stratified formations, throw them out of their horizontal position, overflow them in part, insert themselves among them as intrusive masses, and fill up rents and fissures in the form of dykes and veins. All this gives ample evidence of former change, and presents a lively picture of the operation of these gigantic forces which are still so instrumental in modifying the existing aspects of our planet. In fine, the whole crust is replete with evidence, physical and vital, of the earth's former conditions. Our globe writes, as it were, her own history—every layer of mud and sand laid down in water, every shower of ashes or sheet of lava ejected from a volcano, every stem and twig, every shell and tooth and scale preserved in sediment, forming an incident in that progress which it is the great object of geology to unfold. Geology is in fact the Physical Geography of former ages. For just as the geographer endeavours to depict the existing aspects of sea and land, the climates they enjoy, and the plants and animals by which they are peopled, so the geologist labours to recall the aspects of the past—the distributions of sea and land at each successive stage, the plants and animals by which they were characterised, and by inference the nature of the physical conditions, genial or ungenial, by which they were surrounded. The methods of the one are but the methods of the other; and the more the geologist knows of the existing operations of nature, the better will he be able to interpret the operations of the past. The phenomena of the present are patent, and for the most part explicable; those of the past are obscure, and, in proportion to their distance and obscurity, the greater the interest excited and the ingenuity required for their interpretation.

And surely if men take an interest in the history of their own race—in the mounds and barrows, the tombs and pyramids, the towers and temples of bygone populations, whose dates extend at most to a few thousand years—much more ought to be their enthusiasm in that higher history which carries the inquirer from the historic to the prehistoric, and beyond the prehistoric into events and aspects whose distance can only be indefinitely indicated by eras and cycles. The events of the one history are scattered over a small portion of the earth's surface, and for the most part only under a few feet of rubbish; the events of the other are universal, and found in every stratum that enters into the composition of the rocky crust. The events of the one history are no doubt more direct and immediate; but the remoteness of the other, their strangeness and their variety, should only excite our interest the more, and exalt our conceptions of that Creative Wisdom which has exerted itself in this world of ours ages before the human race became witnesses of its beauties or participators in its bounties. And the more minute our analysis of the earth's crust, the clearer we can render this history; the closer the connection we can establish between the successive stages of its formation, the more attractive and instructive will the geological record become.

In arranging the rock-formations of the earth, we may either divide them, as the older geologists did, into Primary, Secondary, and Tertiary; or looking, as modern geologists do, more especially at their fossils, we may adopt the subdivisions, Eozoic, Palaeozoic, Mesozoic, and Cainozoic. In either case these main divisions contain several formations of marine, estuary, or lacustral origin, and these it is customary to name either after their prevalent rocks, their most characteristic fossils, or some geographical area in which they are typically displayed. Thus the Cretaceous or Chalk system is so named from chalk-rock forming its most distinctive feature in the south of England, and the Old Eed Sandstone from its consisting largely of reddish-coloured sandstones; while the Silurian is named after the district between England and Wales where it is typically developed, and which was anciently inhabited by the Silures, and the Laurentian because typically displayed in the region of the St Lawrence. Adopting this plan (and it matters little what the nomenclature, provided we be certain of the chronological order), the stratified rocks of the crust may be arranged in the following manner—not going into minutiae, but simply presenting such leading features as may convey to the miscellaneous reader some idea of the sequence that prevails among the stratified systems, and the ascent of life, vegetable and animal, as it makes its appearance from the lowest to the highest formations :—

CHRONOLOGICAL SCHEME OP THE EARTH'S CRUST.

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Plants and animals still easting;
a few genera recently extinct.

(Sands,

<> coral-reefs, and other recent superficial' I accumulations. J

[Stratified clays, sands, marls, various) Plants and amlnals °f existm& °/der^ i a

'< large proportion, however, of extinct

(, genera and species.

[ Stratified clays, sands,

( limestones, beds of lignite, &c.

{Chalk with and without flints, calcareous
clays, liguites, greensands, and various
sandstones.

(Calcareous sandstones, sandstones, clays
( shales, coals, ironstones, and limestones
( Reddish and variegated sandstones, marls,
\ pebbly beds, and shelly limestones.
( Reddish sandstones and conglomerates,
( magnesian limestones.
j Sandstones, shales, fire - clays, coals,

< limestones, and ironstones, in repeated
I alternations.
(,Various coloured sandstones (often red),

< conglomerates, marly shales, and con.
(. cretionary limestones.
( Slaty beds, limestones, shales, grits, and
\ sandstones.

(Slaty beds, crystalline schists, grits, and
I conglomerates.

( Crystalline schists, limestones, and ser( pentines.

("Marsupial mammals, birds, reptiles,
J fishes, shell-fish, Crustacea, zoophytes;
palms, coniferse, ferns, lycopods, sea-
(, weeds.

(Marsupial mammals, birds, reptiles,
J fishes, shell-fish, Crustacea, zoophytes;
j palms, cycads, coniferse, ferns, lyco-
V. pods, sea-weeds.

(Reptiles, fishes, shell-fish, Crustacea,
zoophytes;coniferse, ferns, lycopods,
sea-weeds.

(Fishes, shell-fish, Crustacea, zoophytes;
ferns, lycopods, sea-weeds.

j Shell-fish, Crustacea, worm-tracks, zoo

\ phytes; sea-weeds.

( Crustacea, worm - burrows, and zoo

\ phytes.

j Traces of lowly or foraminiferal organ

\ isms.

Studying the preceding scheme, it will be seen how numerous are the formations that compose the earth's crust, each formation representing the sediments of former lakes, estuaries, and seas, and each varying in composition according to the conditions under which it was deposited. It will further be seen that from the oldest to the most recent there has been an ascent, in general terms, from the lower to the higher forms of life—the sea-weed preceding the fern, the fern the conifer, the conifer the palm, and the palm the true exogenous timber-tree; and so in like manner the zoophyte preceding the shell-fish, the shell-fish the fish, the fish the reptile, the reptile the bird, and the bird the mammal. We have thus revealed, by a study of the earth's crust, what our forefathers never dreamt of—namely, that this crust is in a state of incessant change, what was formerly dry land becoming the sea-bed, and what was once the sea-bottom being upraised into dry land; that these old sea-sediments constitute the formations which compose the earth's crust; that these formations are replete with the evidences of former life; that this life evinces a progress from lower to higher forms; and that all the interchanges of sea and land, all the waste and reconstruction, all the growth and decay of bygone life, establish an antiquity for this world of ours vast beyond all human conception, and only to be vaguely expressed in epochs, and aeons, and cycles.

Summing up, then, our knowledge of the rocky crust— and this without any conjecture as to the nature of the earth's interior—it may be stated in general terms, first, That this rocky crust is in a state of slow but ceaseless change, and that the causes—meteoric, aqueous, igneous, chemical, and organic—that now waste and reconstruct, have been productive of similar changes in all time past. Second, That these changes, like all other natural

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