undergone many changes; it has been formed into rocks, and again turned into soil, by their decay; changed again into rock, and again into soil, and this has been taking place over and over again, as our various geological formations were being built up. It is as if some materials of a building had been repeatedly used in the reconstruction of some more recent portion, and that these had been again partially employed in a similar manner. The earliest soil produced from the primitive rocks in like manner has become rock and then soil, and again rock and then soil, very many times. In the periods during which it was existing as soil, it may have been intermingled with the soils produced from other primitive rocks, but of a somewhat different composition, and thus its character may have been changed. The changes which have been taking place have been a series of reconstructions of the same original earthy matter, and these have often been modified in character by the influences of animal and vegetable life. Thus the soil of the field has an antiquity of no ordinary character, and it has probably taken part in many of the great changes on the earth's surface. This is beyond doubt a matter of very deep interest, but there is one practical issue which we must not overlook-viz. that as these reconstructions have been taking place in the manner referred to, we ought to find in the primitive rocks materials similar to those which we have in the soils of our fields. These primitive rocks were the first grand store of solid mineral matter,

the magazine from which all the supplies were drawn, and the various rocks which were found at latter periods of time were little more than reconstructions of the original materials of the primitive rocks. As a matter of fact we do find in the primitive rocks just those mineral matters which now exist in our soils.

These primitive rocks practically consist of three tolerably distinct groups, which are known as Granite, Syenite, and Trap, and their composition is as follows:

If we notice the minerals each of these contains, and trace out their composition as shown by chemical

analyses, we find these primitive rocks differ greatly in the materials each would yield upon decay. Thus the original earthy matter varied very greatly in composition and character; and according to the proportions in which these happened to become mixed, so was the character of the reconstructed rocks modified. So also in the soils which were produced by their decay equally marked variations were to be found. In this way we have immense variations in the character and composition of the soils now existing on the world's surface.

It is desirable, as we proceed, to notice how the decay of rocks into earthy matter is accomplished. We have seen that ordinary stone buildings bear signs of the same decay which took place in the early period of the world's history on the first rocks on the earth's surface, and which continues to take place at the present time. The agencies are very simple, and easily understood. It is, moreover, exceedingly important that these agencies should be familiarly known to the student of Agricultural Science, because they are valuable friends to the cultivator of the soil, and it is always desirable to be acquainted with our friendly helpers.

Those who have watched farm work have probably been often struck with the bright polished surface of the mould-board of an iron plough after it has been used on some of our soils, and especially on sandy soils. When the work is completed, and the plough is allowed to stand out in the field, the bright polished surface soon becomes covered with rust. This rust

is caused by something in the air, which attacks the iron. Chemists are able to trace out what it is in the air which does this, and they find that it is a body which is commonly known to us by the name of oxygen, which is really at the bottom of the mischief. We also know that this oxygen has a companion in the air which also helps on the work, and it bears the name of carbonic acid. These two gases-for they are as colourless and transparent as air are found to help each other very much; and they not only attack metals, such as the bright mould-board we have referred to, but they attack rocks, however hard and tough they may be. If you rub the rusty surface of the mould-board, you can remove a powder, and this shows that the hard metal can be reduced to a soft powder. That powder contains iron, which has been separated from the iron mould-board by these agencies, for even its great strength had failed to resist their attack. If that iron had been beaten or crushed to the greatest possible extent, it could not have been as completely powdered as it was done by these two gases, without any noise, and almost without observation. Thus, although we cannot see these two bodies, we easily learn that they are very powerful, and it will cause no surprise when we say that they are equally powerful in breaking down rocks, even when those rocks are as hard as granite.

CHAPTER IV.

THE changes of temperature and the action of water are another pair of agents which work together in helping to convert rocks into soil. As we have already noticed, the two gases in the air helped forward the breaking down of metal and rock into fine powder, and the work was made more effective by their united action. It is just the same with these additional agents; by working together these also become increasingly powerful. It may be well just to notice how a hard rock is attacked, and how the surface is removed and crushed to powder.

We will take the hardest granite rock, and see how they attack it. It happens that the granite, with all its strength, has two bodies in it, which are really prepared to act as traitors in the camp, and these bodies are iron and potash. Both of these substances are commonly known to us all, and we have therefore a certain familiar acquaintance with them. The iron in the granite is soon found to enter into alliance with some of the oxygen and carbonic acid of the atmosphere, just as happened in the case of the iron mould board of the plough, and after a time we see on the face of the granite a rusty mark. This rusty mark has been caused by the rain-water removing the rust formed on the face of the granite. In fact we have here an exact repeti