THE CRUST OF THE EARTH.

SKETCH OF GEOLOGY.

P. Last week we finished the history of the Secondary Rocksthe beginning of the sixth daywhen the creeping things, the giant reptiles, were the masters of the earth.

W. And the last rocks you spoke of were the Chalk Rocks, which make such great hills in the south of England, and such tall cliffs at Dover and Hastings.

P. These chalk deposits were in some places of immense thickness. To-day we shall talk of the next set of rocks-the Tertiary Rocks. Ion. What does that mean, papa ?

W. Why, that means third set.
Primary means first set;
Secondary means secondary set;

and

Tertiary means third set.

P. In the sixth day the tertiary rocks, and the soils above them, were formed. At this time the earth must have been in a very rough state. I told you how it was all hot burning matter at first, and had taken all this long time to cool. The fire underneath, too, had begun to be quiet, but it left the marks of its work. There were not only the high mountains which it had raised up, but between the mountains there were great holes; you may call them "valleys," if you like, or "basins."

These hollow basins were famous places for the water to make new rocks in, and in these basins the tertiary rocks were formed.

Ion. And that was a very good thing, because they filled them up, and made the earth smoother.

P. Yes; and therefore the tertiary rocks are not spread over so

wide a surface; they are only found here and there in basins.

At the south of England, near the river Thames, there is a great basin full of tertiary rocks, and on these rocks is built a large and splendid city, called LONDON. Did you ever hear of it? L. Oh, yes, papa.

P. So this basin is called the London basin. PARIS, too, is situated on a basin, which is called the Paris basin.

I have brought you some of the strata found in these basins. Here is a piece of the first great layer. Take it in your hand.

L. How soft it is! Is it clay, papa?

P. Yes.

L. But I have never seen such blue clay before, and I can easily make it into any shape. See!

W. So it is plastic. We learned that word in our lesson on clay. I shall call it the stratum of blue plastic clay.

P. That is its name. Here is a piece of one of the next strata in these basins.

L. This is quite white, like chalk.

P. And here is a yellow piece, and here is a grey piece. This stratum is formed principally of lime, just as chalk is; but we do not call it chalk, for it is rather different.

Ion. What is it called, papa? P. It is called gypsum. Great quantities of it are found in the Paris basin; so it is sometimes called "plaster of Paris."

W. Oh, I know what that is; it is the stuff they use to make images with. What is above the plaster of Paris?

P. There are several layers; the principal is a layer of rock, which seems to be made of little grains

of sand sticking together. It is called sandstone.

Ion. So that the principal layers of tertiary rocks are-the blue plastic clay, plaster of Paris, and sandstone. Pray what is above these, papa? Fourthly rocks?

P. No; after the tertiary strata we are very near the surface of the earth. It seems that soon after these rocks had been formed, there were many great deluges. Very violent deluges, too, they seem to have been. The rushing waters seem to have broken off great masses of rocks from the mountains, and even to have washed away great parts of the mountains themselves. These deluges must have been very violent; they knocked these pieces of rock together for a very long time, and rolled them backwards and forwards until they were quite rounded.

After the waters had passed away they left a sediment of clay full of these large stones, which we now call boulders.

W. Ah, I remember! You told us about those boulders in one of our old lessons.

P. So I did. And above the boulders is a layer of smaller stones-perhaps the chips which were broken off when the boulders were being rounded. There are some pits of this layer in the fields very near us. They are called gravel-pits.

Ion. To be sure-gravel is full of little stones.

P. This layer of gravel and sand was not formed by the seabut was washed down from the earth by the rivers.

L. What are the names of these two strɛ 'a?

P. It hardly right to call them strata, a they are not spread over

the earth in regular layers. We call them accumulations.

The accumulation of clay and boulders formed by the sea we call DILUVIUM-and,

The accumulation of gravel, and sand-mud, and other odd things, washed down by the rivers is called ALLUVIUM.

Then, above the alluvium is the soil on the very surface-this you may remember consists chiefly of clay, flint, and lime, with decayed vegetable and animal substances. It is called vegetable soil.

W. Hurrah, papa! And now we have reached the surface again. I am so glad, for I can breathe much better-we have been such a long time poking underneath the ground.

Ion. But the rocks were a much longer time in being formed, Willie.

P. Indeed they were. Oh, how long, none but the Great Maker can tell. From the time when the burning "mass of matter" began to cool, to the time when it was cold enough for man to live upon, immense periods passed away, too vast for us to reckon. To form the Primary, Secondary, Tertiary rocks, and the "accumulations," must have cost perhaps many hundreds of thousands of years."

L. And now, papa, that you have told us about the minerals, will you describe to us some of the vegetables and animals that lived then?

P. Not to-day. We might stop to talk of many more things, but there will not be time. Here is a rough drawing for you of some of the strata in their proper order. I must, however, remind you once more, that although they are found placed in this order, they do not lie above one another so regularly; some of the strata lie obliquely,

and some-I think I said so before -are almost perpendicular. They seem to have been shaken by the fire into all manner of positions. I hope that you want to remember

their names, and will have the good sense to learn them carefully. Next week, if you know them properly, we will talk about the vegetables and animals.