without this presumption the history of the Past would be an uncertainty and delusion-the geologist proceeds to unfold the history of the stratified deposits, tracing back from the silt of yesterday's tide to the first-formed strata; and this through the lapse of ages for which chronology has no name save "cycles" and "systems" of indefinite duration. Geology is not entitled-it dare not, in the spirit of true philosophy, appeal to "abnormal conditions," to "cataclysms," or to "revolutionary forces," for a solution of its problems. Certain agents may act over certain areas with greater intensity at one period than at another, or may exert themselves, in the varying distributions of sea and land, over wider areas; still the results are homologous though differing in magnitude, and cannot be ascribed to convulsion or disorder. Where geology cannot explain, it can at least observe and describe, and this its legitimate cultivators will ever do, rather than take shelter under the assumption of abnormal conditions in primeval nature. There is ever much more philosophy in honest doubt than in the utmost ingenuity of unsupported assumption.

"The agencies," we have elsewhere* observed, "that now operate on and modify the surface of the globe; that scoop out valleys and wear down hills; that fill up lakes, and estuaries, and seas; that submerge the dry land, and elevate the sea-bottom into new islands; that rend the rocky crust, and throw up new mountain-chains; and that influence the character and distribution of plants and animals,— are the same in kind-though differing, it may be, in degree as those that have operated in all time past. The layers of mud, and sand, and gravel, now deposited in our lakes and estuaries, and along the sea-bottom, and gradually solidifying into stone before our eyes, are the same in kind with the shales and sandstones and conglomerates that

* Advanced Text-Book of Geology.

compose the rocky strata of the globe; the marls of our lakes, the shell-beds of our estuaries, and the coral-reefs of existing seas, year after year increasing and hardening, belong to the same series of materials, and in process of time will be indistinguishable from the chalks, and limestones, and marbles we quarry; the peat mosses and jungle growth, and the vegetable drift that have grown and collected within the history of man, are but continuations of the same formative power that gave rise to the lignites and coals of the miner; the molten lavas of Ætna and Vesuvius, and the cinders and ashes of Hecla, are but repetitions of the same materials which now compose the basalts and greenstones and trap-tuffs of the hills around us; the corals, and shells, and fishes, the fragments of plants, and the skeletons of quadrupeds, now imbedded in the mud of our lakes and estuaries and seas, will one day or other be converted into stone, and tell as marvellous a tale as the fossils we now exhume with such interest and admiration." Without this uniformity in the great operations of nature, our reasonings would be baseless, our conclusions a dream. We can only read the Past as connected with the Present, and premise of the Future from what is now taking place around us.

Destroy this belief in the continuous operation of natural law, and appeal to "revolutions" and "cataclysms," and you present a world of disorder, a Creator without a plan, and the human reason striving in vain to elaborate a system from phenomena over which no system prevails. Establish this belief, and the geologist feels he is dealing with a prescient plan whose past ever bears certain appreciable relations to its present; and in tracing the development of that plan, he is animated by the high hope of ultimately attaining to some conception, however faint, of the divine idea of its Creator. And it is in this spirit of procedure that he has subdivided the strata of the earth's crust into "sys

tems," and "groups," and "series"-each system being but the sediments of the lakes and seas of a certain period, and characterised, of course, by its own peculiar fossils, as evidence of the life that prevailed during the time of its formation. And the reason is obvious: as land and sea have often changed places the former at one time more insular, at another more continental; now sitting low and moist in the water, now elevated into lofty and arid regions; subjected at each change to diversity of colder or warmer ocean-currents, to new sets of winds, rains, and other climatal conditions-each period must necessarily have stamped its own impress on vegetable and animal life; and so it happens that the great rock-formations (the only records of the world's history) are each characterised by its own peculiar fossils, or facies of animated existence. Thus, when tabulated, these systems and groups present the following chronological arrangement :—

Such are the main stages into which geologists have ar

ranged the stratified crust of the globe-the great chapters, as it were, of world-history, whose strata, like the leaves of a mighty volume, are indelibly stamped with the forms and characters of extinct vitality. As in human history we speak of the times of Ninevites, Egyptians, Greeks, and Romans, so in geology we refer to Silurian, Devonian, Carboniferous, and other systems; and as Ninevites and Egyptians present a certain similarity or facies of civilisation, and Greeks and Romans another, so we unite certain systems, having features in common, into Palæozoic, Mesozoic, and Cainozoic epochs. As to the Time represented by these groups and systems, we have at present no means of determining; but, gauging the past by the present rate of geological change, the amount must be immense, and we could no more form an idea of its aggregate—even could we express it in years and centuries-than we can form a conception of the distances that separate our globe from the remoter stars of the universe. Enough for us, in the mean time, to be convinced of the vastness of its relative portions, and to fix with certainty the order of their occurrence. As in human history it is ever more important to determine the true sequence and connection of events than to be curious about the minutiae of dates, so in geology it is far more satisfactory to discover the order in time than to indulge in surmises about the expression of its duration in years and centuries. It is surely of higher value to be able to determine the relative ages of two contiguous deposits, the contemporaneity of others widely apart, and the kind and character of life they respectively imbed, than to perplex ourselves with vague hypotheses as to the number of years that have passed since the date of their deposit. And yet even for this, too, the time will undoubtedly arrive! Geological events are the orderly results of natural laws; laws are as fixed in their times as in their modes of

action; and while the Creator has permitted the human intellect to investigate and determine the one, we may rest assured that the same intellect is yet destined to discover the amount and duration of the other. In the mean time, all that geology attempts is to arrange the formation of the earth's crust into so many provisional stages—each stage representing an indefinite amount of time, but embracing such stratified deposits as indicate a contemporaneity of origin, and are characterised by a general similarity of organic remains. In this case, each stage represents the sediments of a certain period, and is necessarily characterised by its own peculiar fossils-every change of sea and land not only giving rise to new sediments, but to altered conditions of vital existence, that are inevitably followed by a modification of the flora and fauna. And summing up the whole, we are presented with the outline, at least, of a grand and continuous evolution of vitality. Here there may be local imperfections in the record-there the characters may be fragmentary and obscure; but in the main the broad features of world-history are sufficiently obvious, and these systems and formations (provisional as they may be) enable the geologist to give intelligible expression to the line and order of occurrence.

Proceeding upon the basis of this arrangement, let us now inquire into the nature of the Plants and Animals preserved in these successive formations. Were they constructed on the same plan, and destined to perform analogous functions in the economy of nature, with those that now live and flourish around us? Or if differing in type, what the amount of that difference, and the presumable function which that difference implies? If race after race has come and departed, what the conditions that accompanied their advent, and what the causes which apparently