and other particulars of the physical characteristics of many of the bodies revolving around us are known. We could not attain to greater practical certainty with respect to these if they were terrestrial bodies, and measurable by line and balance. It can be said with certainty, for instance, that a planet cannot be more than nor less than so many thousands or millions of miles distant from us, and that it cannot be more than nor less than so many times heavier than a globe of water of equal size. It cannot, however, be affirmed of any rock belonging to our earth that it is not more than so many years old; but it can be said in many cases that it is not less than a given number.

Geologists have come to the general conclusion, from a vast number of facts all pointing in one direction, that some rocks are much older than others, that some were formed comparatively rapidly, and that any one of the series indicates the lapse of a period of time compared with which all historical time is insignificant. Thus any one of the great limestone series of rocks probably occupied a longer time in formation than the whole period of man's existence on our planet. It is difficult to realize these intervals of time, but we can obtain a general idea of them thus: We draw a horizontal line on a board about one-sixth from the top, and another about one-third from the top. The lowest space represents the Primary and Plutonic, or lowest rocks, of unknown thickness; the next the Secondary or middle rocks; and the upper the Tertiary, or latest-formed rocks -designations adopted for the sake of convenience to

denote eras which are not, however, marked by any well

defined boundary lines.

TERTIARY,

or Upper Rocks.

SECONDARY,

or Middle Rocks.

PRIMARY,

or Lowest Rocks, of unknown thickness.

The two upper may be, roughly speaking, from fifteen to twenty-five miles thick, taken together; and the whole of these consist of the detritus (worn material) of the older

rocks, with animal remains originally deposited in seas and fresh-water lakes, etc., and of masses of compressed vegetation. Does it not appear, then, on the face of it, that the formation of these latter rocks must have occupied an enormous time? We are dealing with no hypothesis here we are on the ground of indisputable fact. These latter rocks have been measured. This must

be taken for granted now. It will be explained subsequently how it has been done.

By a simple but thoroughly scientific process it has been computed that many millions of years have elapsed since the first of these Secondary rocks began to be formed. We need not enter into particulars now, but it may be explained shortly how this has been arrived at. It can be ascertained with tolerable certainty, and has been done for many of the great European rivers, what quantity of solid matter-worn rock, material, and vegetation-is brought down annually by a river. Working together the data for England and Wales, it has been shown that the rainfall over that area would occupy 13,000 years in removing from the surface a single foot in depth of earth-to be carried away and deposited elsewhere.

None of the calculations of these periods is exact probably. We can only say that they are, in many instances, more likely to be under than beyond the truth. All such estimates are of necessity founded on the rate of wear and tear going on now, because we have no other means of measuring the action of water; but we must remember that there is evidence of much more

rapid water action in some past times. The impression still remains, however, that no such limit must be placed to the age of our earth as we were accustomed to assign to it before geology became a science, and the facts collected were arranged in order by the great teachers of it.

There is another form of evidence for the age of these deposits, which we may call the organic proof.

Chalk (one of the Secondary rocks) is composed of (chiefly) carbonate of lime, mainly derived from the dead and fossil bodies of small shell-fish called foraminifera, from the number of chambers in their shells (Latin, foramen, a chamber), and corals. Now exactly the same process of formation of Secondary rocks is going on today in the great oceans, notably the Atlantic and Pacific, and even in high latitudes. Incalculable numbers of these little creatures are living at or near the surface, where they assimilate the carbonate of lime suspended in the water, die, and shed their minute shells upon the floor of the ocean. A grey ooze or mud is thus formed, in all essentials identical with chalk, and it is the chalk of the future. In the chapter on sedimentary rocks the process of deposition will be more fully explained. We must surely allow an immense time for the deposition of rocks in this manner, hundreds of feet thick, and distributed all over the world.

According to their structure, all rocks may be divided into two great classes-the unstratified and the stratified. The unstratified have been formed, to a great extent or wholly, by the action of fire, and include the granites,

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lavas, etc. These show no sign of having been deposited in water, and contain no organisms "—i.e. nothing which ever possessed life, either animal or vegetable. They are more or less crystalline in structure, are excessively hard, and do not split easily. The unstratified rocks, which are at the bottom of the lowest division, are the oldest of all; and this will be apparent when we think of their origin.

Astronomers tell us that the earth is possibly a portion of the sun detached from the parent mass and hurled into space, and now revolving round the sun in its own orbit. It was, doubtless, at the earliest stage of its independent existence, in a state of intense heat, and its materials in a molten, if not a gaseous, state. As the new planet gradually lost heat, the surface contracted, and the rock material crystallized into a form which may, perhaps, be represented by granite. We may regard the earth at that time as consisting of a globe of molten material, covered with a crust of rock which had cooled down enough to become crystallized. This has since undergone great changes, by which the present beautifully diversified contour of the surface has been brought about. Greatly as the earth must have cooled down since its separation from the sun, there is still plenty of evidence of the present existence of intense internal heat. The volcanoes in various parts of the world testify to this; and the lava they cast up, and which runs down their sides, in some cases forming rivers many miles long and broad, and hundreds of feet deep, is melted rock matter derived from the mass