Cooling of the Atmosphere. 187 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 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 the 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 greatly differ 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 as it is now. 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 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. Refer for their refutation to the lecture on chemical geology, by the late 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. The mean specific gravity of the earth is 5'45; 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 changes have been wrought by agencies similar to those now in opera tion. 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 carbonic acid, excessively present in the primeval atmosphere. In process of time, the actions of rivers and seas arranged the comminuted particles in sedimentary beds of varying density and character. 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. The 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. Human intelligence can assign a relative age for the igneous, aqueous, organic rocks, and state when the plains and mountains were formed. The various strata are not necessarily separated by vast intervals of time; some were of great horizontal extent, some were formed synchronously. For illustration of the history as to rocks, take up a piece of granite-you discover that it was once molten within the earth under enormous pressure. Find a sandstone, it teaches you that small pieces of matter were worn out of pre-existent rocks, then compressed together and hardened. If we have a lump of chalk, and gently grate or knead it down in water, we see that it consists 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, belongs to what are called "Organic Rocks." Coal is so much vegetation pressed together, and gradually chemically 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, with remains of star-fish and other creatures. If they continue undisturbed, a vast mass of rock will be formed out of these dead. The hills and dales of Derbyshire Lifetime of the Earth. 189 and Yorkshire are chiefly formed of limestone: the crowded remains of little animals which peopled the waters of the sea. These fossils, or relics, generally enable us to know the origin and age of the various and wonderfully arranged strata which form the crust of the earth. Geologists define the lifetime of the earth as Eozoic, dawn of life; Palæozoic, old life; Mesozoic, middle life; Neozoic, new or modern life. 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 of vegetable life derived from the limestones, graphite carbon, and iron ore; 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 known 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, called Azoic, contained no vestiges of life, and that the ocean then existing was lifeless; but the Laurentian contains a gigantic representative of the earliest known life on earth, the Eozoön 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 Eozoön. The Paleozoic, old life, has its own ages and variety of strata Laurentian, pre-Cambrian, Cambrian, Silurian, Devonian, Carboniferous, Permian. The lower two present 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 and amphibians. 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 distorted and exaggerated as if they had outgrown themselves. The coal measures present remains of vegetables, insects, land-snails, fishes, amphibians-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, mineralised. In the vast Palæozoic duration, if we reckon thickness, nearly nine-tenths of all the known rocks were formed in the earth's crust. A definite plan, even in these early ages, seems to have associated life with physical and organic change. The Mesozoic, middle 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 classes 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, flew in the air. There was abundance of a particular group of plants, still represented in the tropics and Austràlia; and the great forests of the later Mesozoic, gay with flowers, beautiful in foliage, swarmed with insect life. Gigantic lizards were remarkable, exhibiting a more generalised type of reptile organisation than any now existing. Pterodactyles, somewhat like great bats in shape, wheeled and screamed in the air, pouncing on smaller creatures of their kind, and perhaps diving into the sea for Reptiles and Mammals. 191 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, Lias, Oolite, 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 onefifth or sixth, of the time taken up in the Palæozoic period. The Neozoic time, the great age of mammals, is subdivided into Eocene, Oligocene, Miocene, Pliocene, Pleistocene periods -every one of which witnessed the production of a formation -were so named in consequence of their containing different percentages of species which still exist. In the Eocene the percentage is least, in the Pliocene the greatest. 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 many 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, noble forms of animals. The Pliocene abounded in species of elephant, rhinoceros, hippopotamus, and horse, now extinct. There are abundant traces of oxen, deer, 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. The Pleistocene is remarkable for the advent of man. He seems to have had his first dwelling in the East-a pleasant land, where flesh food was not strictly necessary for him. The history of the earth is wonderful. The consecutive formation of continents, deep oceans, mountain ranges, indicates repeated upheavals, subsidences, curvings, caused by |