BASALT.-HORNBLENDE.-ANGITE.-OLIVINE. 35

it is well-placed, I think, beside this piece of coal.

MRS. R.

The sombre-looking specimen is called Basalt, which forms some of the most singular rocks in nature: such as the Giant's Causeway, in the north of Ireland; Fingal's Cave, in the Island of Staffa; besides many romantic cliffs and mountains in various other parts.

EDWARD.

Is that a crystal of hornblende in it? I think it resembles what we were examining.

MRS. R.

Yes, that is Hornblende; but the other crystals which you see here are different: this one is called Angite, and this other Olivine. It will be proper for you to remember these, as they are frequently referred to in discussing the origin of basalt.

EDWARD.

This specimen, I should suppose, you call slate basalt, as it has a very similar appearance?

36

SHALE, OR SLATE-CLAY.

MRS. R.

No: that is called Shale, and sometimes SlateClay, to distinguish it from the clay-slate which we have already examined. It is usually found in connection with coal, and containing the remains of plants as well as shells.

EDWARD.

Well, I should like to know something more about this shale; and the coal, too, must be interesting, since it is so useful to us.

MRS. R.

We shall have a great deal to tell you about the origin and formation of coal, when I come to explain the several systems of Geology.

CHRISTINA.

Here are two pretty pieces of square crystal; you do not surely call these rocks?

MRS. R.

Not rocks, when they are in such small pieces; but one of those often forms considerable masses. If you touch it with your tongue, I dare say you will be able to say what it is.

ROCK SALT.-CALCAREOUS SPAR.

37

CHRISTINA.

Oh, it is quite salt: I thought it was hard, like crystal.

MRS. R.

That is Rock Salt, which is also an important substance in systems of Geology; the other is crystal of Calcareous Spar, as it is called, and is composed of lime.

EDWARD.

Are these all the rocks which we have to know in studying Geology?

MRS. R.

These are all that are properly called rocks; but, besides these, you should be acquainted with the substances that usually cover the surfaces of rocks, which are called Alluvial substances: such as gravel, clay, sand, and the round stones of all sizes which are found scattered over the ground, called, by Geologists, Bowlder-stones; but all these you will become best acquainted with as we proceed when we shall see, also, how gravel, sand, clay, and bowlder-stones, are formed from the original rocks; and how even some of the

38

ORDER AND FORMATION OF ROCKS.

newer rocks themselves are formed from the older ones. In the meantime, you must spend a day or two in going over your specimens in the cabinet attentively; and, as soon as you know them well, I shall tell you something of the wonders of Geology.

EDWARD.

But, may I ask, mother, if the rocks are always placed in the same order as they are here in the cabinet: granite first, gniess second, mica-slate third, and so on?

MRS. R.

The reason, my dear, of placing them in this order, is, that granite is in nature usually found to lie the lowest ; and upon it, as a foundation-stone, gniess, mica-slate, and the other rocks, are seen to rest; for the most part, in the same order as they are placed in the cabinet.

EDWARD.

They are not always, then, to be found in the same order? I fear that will be rather puzzling to us in understanding them.

MRS. R.

When Geology began to be studied, it was sup