beneath, which has probably never before been crystallized. We may consider volcanoes the outlets of the expansive force of the internal heat, and, as it were, the safety-valves without which earthquakes would cause vast rents in the surface. Many volcanoes have long ceased to emit lava, and some have become covered with stratified rocks. All this indicates a gradual loss of heat, because so many of its outlets have become closed. But from the appearances we observe it is possible to form a very clear opinion of the forces which have operated in former times. Destructive as these fires are even now whenever they break forth; while the unstratified rocks alone existed the surface of the earth must have been in a continual turmoil with explosions of steam and gas, and shaken with mighty earthquakes. Fortunately these convulsions are now limited to a comparatively small area, otherwise vegetable and animal life would be impossible. The gradual diminution of this violent eruptive force prepared the way for the action of water in forming the next series of rocks— the stratified at whose history we may now just glance.

We have passed from the more violent periods of geological action, when the forces were mostly derived from within the mass of the earth, to those quieter times when the outward influences of air and water worked changes not less mighty and far more interesting, when vegetation began to clothe the surface, and lowly forms of animal life inhabited the salt and fresh waters of a world progressing towards order, organization, and beauty.

The stratified rocks were formed from the unstratified by various physical agencies, which will be described subsequently. We cannot too firmly impress this upon our minds, that the unstratified rocks have been the parents of all others. What a period of time this opens to our view! All that fifteen miles or so of Secondary and Tertiary and a great part of the Primary rock has been built up of the worn particles of the lowest series! Water, air, heat, and cold have acted upon them incessantly, until an immense accumulation of detritus has resulted, and this has been arranged and rearranged by oceans and rivers, and worked into a form capable of supporting life. The stratified rocks, then, are the sediment of the unstratified, and among them are the specially organic or "life-formed" rocks. They are called stratified, from the Latin word stratum, spread out; and they are usually capable of being split in certain directions. This is the "line of cleavage," and slate, sandstone, and chalk are familiar examples of stratified rocks. They have generally been laid down in water, and almost all of them contain animal or vegetable remains in more or less abundance. Included among them is coal-the fossilized remains of dense forests which usually grew in marshy situations.

From these rocks we learn the marvellous history of life. They are the records, which nothing can falsify, of a steady progress under eternal laws from lower to higher forms of being. Any one of us with sufficient diligence can read these records. They tell us that the earth has been the scene of life

and death, pain and

C

pleasure, for incalculable ages. The plan has ever been the same-immutable as the laws of matter-but it has been expanded by gradations, always, as far as we can judge, tending towards a higher order of things. Geology tells us, in unmistakable language, that the land and water have changed places repeatedly, that continents have sunk, that oceans have been filled up, that both inorganic and organic rocks have been raised into mountain chains, that there has been a long succession of forms of life appearing and disappearing through cycles of time whose vastness we cannot fully comprehend. Thousands of years must be as seconds of time to him who would compute the earth's age, and whole species and genera of plants and animals are but so many finely graduated marks on the great scale of lifeduration.

It will naturally have occurred to the reader to ask, "How have the relative positions of rocks been ascertained?" The question cannot be answered in a few words, but it will become more plain as we proceed. There are two kinds of evidence for the succession of the rocks-or, taking several kinds together, "formations" -the natural and the artificial. Nature has herself opened the book of geology, and shown us the arrangement of its pages. Rents and fissures of great depth and extent have occurred in the surface; the ocean has battered away portions of the shore, leaving cliffs exposed; rivers have cut deep channels through the soil; the melted rocks have been thrown out by volcanoes; and in these situations the arrangement of the formations

can be more or less well observed. By means of mines, well-borings, and cuttings for railways, canals, docks, etc., we have made sections of the earth at various points, and much valuable knowledge has been gained by these artificial works. We find considerable irregularity, it is true, in the layers, or "strata" as we shall now call them, thus exposed to view, and they are at some spots much twisted, tilted, and broken, and frequently overlap one another; but by observing a great number of these natural and artificial sections, we obtain a very fair general idea of their relative positions. If we call the formations, from the lowest upwards, A, B, C, D, etc., we may find any one of them wanting at a particular spot, but we shall not find D below C or A above C, though some of the lower may have broken through the upper, and there may have been other accidental displacements quite independent of the order of their formation. For instance, calling granite A, it has broken through B, C, and D at many points all over the world, but its base still lies beneath the others. There have also been subsidences of the strata, and then the edge of one rock may have been pushed over the other; but it is always possible to ascertain on examination that the older rock has been forced above the newer, and was not formed there. The natural order is never inverted.

We know of no part of the earth where a section would show the formations complete. One or more would probably be wanting; but it can be safely affirmed that, if we went down deep enough, granite or rocks of like character would be found at the bottom of the series

at every point on the globe. The order of succession has been deduced from comparison of a great number of observations made in widely different places, and thence has arisen a classification of the great divisions, formations (or groups of rocks), and individual strata, upon the general correctness of which geologists are quite agreed.

The geologist deals with the history of the earth in much the same manner as the historian treats that of mankind. Each classifies and groups the subject under periods and systems possessing broad characteristics. Ancient, Mediæval, and Modern in human history thus correspond with Primary, Secondary, and Tertiary in geology. The historian traces the gradual development of civilization under various dynasties; while the geologist devotes his attention to the formation of the solid materials of the earth, and describes the living creatures which have successively occupied it. Geological classification is to some extent arbitrary and artificial, but it serves to generalize certain natural conditions, and affords a convenient standard of reference and comparison-at least sufficient for all practical purposes while our knowledge is yet incomplete. With respect to chronology, geology is confessedly uncertain. We are compelled to go back from the present through epochs and cycles of time ever extending into a still more distant past, until figures fail to convey any impression of our remoteness from the beginning. Attempts, based on very careful calculations, have been made to assign an extreme limit to geological time. By one of the most recent of these, Professor S. Haughton arrives at two