nitrogen the vapour of water still higher. Afterwards, this arrangement would be gradually obliterated by diffusion; but it is imagined that, before diffusion, this arrangement had considerable influence.

The cooling of such an atmosphere would condense the vapour of salt, and other chlorides, etc., and cover the solid crust of the earth with a solid layer, it is calculated, sufficient to clothe the entire sphere with a coating of some ten feet in thickness. Then the condensed steam would fall in rain, which dissolving greater part of the salt, would form the ocean. The atmosphere would now contain much less oxygen; and the carbon, in form of carbonic acid, would probably not differ much in composition from what it is now. The exact action, and extent of reaction; the amount of any one element entering into any particular state of combination, cannot be defined. We may say, however, because the earth is so little flattened, it must have been rotating, when it became solid, at nearly the same rate at which it is now rotating. If we add, as the rate of rotation is undoubtedly becoming slower, it became solid not many millions of years since; otherwise, it would certainly have solidified into a flatter shape; we arrive at the conclusion that we cannot allow geologists a greater possible period than about ten or fifteen millions of years.

There are arguments against these views; we will not advance them. Reference to the lecture on chemical geology, by David Forbes, delivered before the fellows of the Chemical Society, 20th February 1868, and to be found in the Journal of the Society for 1868, p. 213, will afford their refutation.

The mean specific gravity of the earth is 5'4; leaving out the water, the mean density of the exterior is not higher than 275 or 3; it follows that the interior is immensely more dense than the exterior. The crust, at first, might present a somewhat even contour; but soon would be crossed by cracks and fissures, caused by contraction of the mass, and portions of the crust would fall in; then protrusions of molten matter formed dykes on the surface. The sides of the cracks being more or less dislocated, lines of faults would interrupt the previously regular contour, and form the first elevations or mountains.

From that time till the present all the

Action by Organic Life.

173

changes have been wrought, it is considered, by agencies similar to those now in operation. The crust, split and broken, would be further broken and pulverised by the mechanical action of water; assisted by the disintegrating and decomposing action of the carbonic acid, excessively present in the primeval atmosphere; and thus, in process of time, the actions of rivers and seas arranged the comminuted particles in sedimentary beds of varying density and cha

racter.

The

The development of organic life, at first of the lowest type, originated another character of deposits. Vegetation, assimilating the carbonic acid of the atmosphere, introduced beds of carbonaceous substance; and animals and plants, by joint operation, built up the limestones and calcareous strata. formation of these strata did not proceed uninterruptedly: outbursts of igneous matter disturbed and broke the surface, forming dykes, ramifications, bosses, and sometimes intercalations between the beds. Showers of ashes were also sent forth from time to time. Any geological primer will explain the nature of the igneous, aqueous, and organic rocks. Human intelligence can assign a relative age for every one, and state when the plains and mountains were formed. The various strata are not necessarily separated by vast intervals of time, and the discovery of a universally existing microscopic vegetation shows, that by agency of the lowest and simplest organisms, our globe might be covered, and probably was covered, with all the chief strata.

For illustration of the history as to rocks, taking up a piece of granite, we discover that it was once molten within the earth. Finding a sandstone, it teaches us that small pieces of matter were compressed together, hardened, broken, rolled in and by water. If we have a lump of chalk, and gently grate or knead it down in water, we discover it to consist partly of microscopic chambered shells belonging to animals of simple form and life-protozoa. This chalk, with all other rocks containing relics, whether of vegetable or animal existence, belong to what are called "Organic Rocks." Coal is so much vegetation pressed together, and gradually changed into the black substance now used as fuel. In the deep parts

of the Atlantic Ocean millions and millions of little shells, called foraminifera, are being deposited; the remains of starfish, and other creatures also leave their remains; if they continue undisturbed, a vast mass of rock will be formed out of these dead. The hills and dales of Derbyshire and Yorkshire are chiefly formed of limestone: a mass of the crowded remains of little animals which peopled the waters of the sea. These fossils, or remains, generally enable us to know the age and origin of the various and wonderfully arranged strata which form the crust of the earth.

Geologists define the life-time of the earth as Eozoic, dawn of life; Palæozoic, old life; Mesozoic, middle life; and Neozoic, new or modern life. These are also called Primary Epoch, Secondary Epoch, Tertiary Epoch, and Quaternary Epoch. They are apportioned into ages and eras of stratific formation.

The Eozoic time is supposed to represent that very period during which the first land was cooled and solidified from the fiery mass. Its rocks are the deepest and oldest, crumpled and folded in a remarkable manner, and the folds appear to have been formed before the deposit of the rocks next in age. There is inferential evidence, derived from the limestones, graphite carbon, and iron ore, of vegetable life, as there are undoubted remains of animal life, but the mystery of the origin of living things, and the secret of the changes which they underwent, remain hidden. The deepest and oldest existing rock or formation, the Laurentian Gneiss, is made up out of the waste of previous existing rocks, but of those pre-existing rocks we know nothing. The Laurentide Hills, north of the river St. Laurence, are the largest known exposure of this ancient formation. They are more than 30,000 feet in thickness, and occupy an area of about 200,000 square miles. It was formerly thought that the lowest rocks contained no vestiges of life, and were called Azoic, and that the ocean then existing was lifeless; but we have found that the Laurentian contains a gigantic representative of the earliest known life on earth, the Eozoon Canadense, the grandest of its class; flashing upon the scene like Melchisedec, without father, mother, or descent of days, the modern representatives

being poor indeed.

We may possibly discover plants yet earlier or an Eophyte period preceding the Eozoon.

The Paleozoic, called also Primary time, has its own ages and variety of strata: Cambrian, Silurian, Devonian, Carboniferous, Permian. The lower present but few traces of living beings, but the upper are crowded with fossils, no longer mere Protozoa, but representative of five orders of life. The sixth also, the Vertebrate, appears in fishes of many kinds. New forms of life come in continually some, without previous representation, appear at once as kings; some, continuing but a little while, find a grave, and are no more seen. These meaner Enochs and Elijahs were not supernaturally removed, for others supernaturally to come; they simply indicate that the natural plan works by continual change. There are kinds which resemble the young of modern animals, but enlarged and exaggerated as had they outgrown themselves. The coal measures present remains of vegetables, insects, land-snails, fishes, reptiles-small and large, prophecies of things to come. In course of this vast time land rose above and subsided beneath the waters several times. There is something grand and awful in the thought of a world of vegetable and animal life-living, dying, slowly carried beneath the waters, and gradually raised again. In this vast duration nearly nine-tenths of all the known rocks were formed in the earth's crust, that is, if we reckon thickness. In the great pulses of the world more and more land was elevated, and the Permian flexures fixed the form of the now existing continents. A definite plan, working through long ages in regular march, seems to have correlated life with physical and organic change.

The Mesozoic, Secondary time, middle period of life, was occupied by myriads and myriads of organisms. On the rocks, formed during this and the Tertiary time, exist the most populous and civilised assemblages of mankind. The movements of the water, and action of the elements, rendered the earth favourable for that kind of vegetation and animal life which man requires. All the lower kingdoms of animal existence were present; birds swam on the surface of the deep, waded in the shallows, left their footprints on the land,

perched on the trees, and flew in the air. There was an abundance of singular plants, still represented in the tropics; and the great forests of the later Mesozoic were gay with flowers, beautiful in foliage, which swarmed with insect life. Gigantic lizards were remarkable, exhibiting a higher type of reptile organization than any now existing. Pterodactyles, somewhat like great bats, wheeled and screamed in the air, pouncing on smaller creatures of their kind, and perhaps diving into the sea for fish. It was the age of reptiles, of mighty and terrible creatures in sea and on land: not continuing till our day, not waiting for man to war against them, they perished in the great cretaceous subsidence. Apportioning this time into the Trias, Qolite, Wealden, and Cretaceous formations, it does not seem to have occupied, so far as we can judge by measurement of the deposits, one-third, or one-fourth, some say one-fifth or sixth, of the time taken up in the Paleozoic period.

The Neozoic or Tertiary time, the great age of Mammals, is subdivided into Eocene, dawn of recent life; Oligocene, recent; Miocene, less recent; Pliocene, more recent; Pleistocene, most recent. Fossils of the Eocene deposits are numerous. Plants, in the main, are closely allied to existing tropical and sub-tropical forms. Nummulites are remarkable, and bony fishes, reptiles, birds, mammals, represent most of the modern orders. The Oligocene formation between the Eocene and Miocene is slightly developed in the south of England, and vastly in the north-east of Italy. There were vast coral reefs in the period, and the varied nature of strata is remarkable for intermediate fauna. The Miocene was, in some respects, a better age than the present. The Northern Hemisphere possessed a mild and equable climate, a vast surface of land, a rich varied vegetation, and noble forms of animals. The Pliocene abounded in species of elephant, rhinoceros, hippopotamus, and horse, now extinct. There are abundant traces of oxen, deer, and carnivora. It is considered that from the Eocene to the Miocene was a time of rapid introduction of new species; but from the Pliocene to the post-Pliocene, and to the modern, there seems to have been a diminution of species. The Pleistocene is remarkable for