hundred millions of years! Obviously a calculation of the age of one formation is more likely to approach the truth than that of the whole series of rocks; because we are dealing with that formation alone, and can place greater confidence in the data derived from and applied to itself than when these data are applied to several other formations of similar but not identical composition. The measures which may be fairly trustworthy in the one instance can only be assumed to be applicable in others. Examples of such calculations and their results will be noticed subsequently. For the present it may be remarked that historical time has afforded very slight opportunities for estimating geological time; but assuredly the former is an insignificant fraction of the latter.

Such is a short outline of the subject, and it will have prepared us to consider the evidence for the now established truths of geology. We shall begin with the unstratified rocks, passing through the stratified series, examining the origin and mode of formation of each, with all the physical forces which have shaped them, pausing here and there to note the fluctuations in the tide of life, and finally arrive at the surface, over which is spread a world of grandeur and beauty-the youngest born of many such lying buried beneath us.

The second part will be devoted to the history of vegetable and animal life upon the earth from the earliest times, and the succession of lower and higher forms throughout the whole series of rocks, illustrated by descriptions and figures of the fossils characteristic of each formation.

CHAPTER II.

THE UNSTRATIFIED FORMATIONS,

Theories as to the earth's origin-Inferences drawn from astronomyObservations with the telescope and spectroscope-Plutonic, igneous, metamorphic, and unstratified rocks generally - Their origin, structure, characters, and position in the series-Affected by contraction, upheaval, and intrusion among other rocks-Consideration of the present condition of the earth's interior, and the forces still in operation beneath the surface.

Ir needs hardly be said that the further back we carry our investigations into the world's history, the less confidently we can rely upon them. Various hypotheses

have been advanced to account for the existence of our planet in the first place, and there are differences of opinion on its early condition. It will be best, however, to avoid discussion of these hypotheses in detail, and consider that alone which appears most probable and has become generally accepted.

Assuming that at some immensely remote time the earth did not exist as an independent body, its derivation from some other and larger mass of matter is the only conclusion possible. Its parent may have been the sun. Our system is so compact and inter

dependent, the sun controlling the motions of all the members of the system, that it is not easy to imagine the earth coming into this system as a stranger from distant regions. It is far more probable that it is a portion of the largest and central body, the sun, than of any other. Or we may consider each member of the system to have been a vastly extended mass, which has gradually contracted to its present size.

The earth, then, in common with other planets, may be supposed to have passed from the condition of a gaseous to a highly heated fluid mass, and to have finally become plastic, and moulded by revolution on its own axis to its present shape. This shape is that of an oblate spheroid, i.e. a globe flatter at the poles than at the equator. Thus, a straight line drawn through from pole to pole, will be about twenty-six miles shorter than a similar line drawn through from one side to the other at any opposite points on the equator; or, more briefly, the polar is shorter than the equatorial diameter. Now this is indisputable, and in connection with it it is worthy of notice, and can be shown by mathematics, that any plastic body revolving on its axis must, and actually does, assume the form of an oblate spheroid. We cannot but be impressed by the significance of this fact. The polar diameter of the earth has been proved, by numbers of measurements of the surface, to be shorter by so many miles than the equatorial diameter; and any material turning on an axis, and sufficiently plastic to allow of its yielding to centrifugal force, must take the same form as the earth

is known to have. Here is a physical law of universal application; and, when we find that the earth is of the form which such a law always imposes, we cannot resist the conclusion that its materials were semi-solid or plastic at some period of its existence.

Some astronomers regard the building up to a considerable extent of the earth by accretion as probable; considering her as a glowing mass of gaseous vapour, giving off light and heat like a sun, she attracted similar masses of matter from space, and gathered these in to her own body while crossing their paths of circulation. Since, as we know, there is now a yearly increase of her volume, amounting to many thousands of tons of meteoric matter so attracted, the quantity received may have been immensely greater in the earlier stages of her formation, when space was, so to speak, more densely occupied by fragments of matter too small individually to be of any importance, but when gathered up by a larger body, sensibly increasing its size.

Passing outside the boundary of our planet, something is gained by observation of the surrounding bodies. The two great instruments used in these observations are the telescope and spectroscope. Beginning with the nearest of the planets, our satellite, the moon; what does the telescope teach us with respect to it? We can look upon the irregularities of its surface as upon a distant landscape. Any building as large as St. Paul's Cathedral would be distinguishable. Accurate charts have been made of the mountains and valleys, and the numerous craters of extinct volcanoes have

received distinct names. There is no appearance of cloud or water in any form; ice, snow, vapour, and seas are absent; and there is no refraction of light; consequently it is inferred that the moon has no atmosphere, or, if it has, it is in no way physically or chemically similar to that which surrounds us. The moon, then, has all the characters of a burnt-out cinder; a dead world without air, water, vegetation, or animal life; subject to a temperature above that of boiling water during its day, and below zero at night. That the moon has, at some immensely distant period, been the scene of intense heat and violent volcanic action, there can be no doubt. It is one of those bodies which, if it ever had a period of fitness for life, as we understand it, has long passed through that, has lost all its original heat, and is now an interesting ruin, with no apparent relation to us, except as one of the controllers of our tides, and a reflector of the sun's light to us during certain nights of each month.

Now we turn the telescope to a more distant object, Mars. Though this planet is some 50,000,000 miles from us when at its nearest, the positions of seas and continents can be distinctly traced, at least on its southern hemisphere; and the snows upon its poles shine with dazzling whiteness, and decrease very perceptibly every year at a time corresponding with our midsummer, increasing again in the winter. Clouds, driven by strong winds, appear on its surface; and altogether its meteorological conditions are very similar to ours. Here, then, is a planet on which life is possible,