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CONVERSATIONS ON GEOLOGY.

CONVERSATION XII.

Fossils-Peat-Jet-Coal-Animal Remains.

MATILDA. I have been impatiently expecting the subject of Fossils to take its turn in our conversations., Now I think the time must be come: for I am persuaded we shall not be able to go much further without some acquaintance with these extraordinary productions.

MRS. L.-I have no objection to devote this conversation to the subject exclusively, though it will intermix itself with most of our future conversations. That you will feel particularly interested in it, I cannot doubt; it will take us a little out of the usual course of Geological treatises, in which it is generally taken for granted that the reader is previously informed; but I consider it quite consistent with our plan thus to turn aside; and indeed, altogether indispensable to you, for the future understanding of the subject.

ANNE. I should like first to be told what a Fossil is. I thought I knew: but the other day, when I remarked to a lady that she had some beautiful Fossils, she answered that they were Minerals, not Fossils.

MRS. L.-In the actual meaning of the word, you were right for Fossil is a term that implies any thing dug out of the ground. But scientifically, it is confined to those substances, that, having once had life, are now found buried beneath the surface of the earth.

ANNE. Would not Petrifactions then be a better word?

MRS. L. This would be still more incorrect, because it implies a substance turned to stone, which in most cases Fossils are not. Many retain their original substance, while others have entirely decayed and been replaced by stone: in neither of which cases can they be

said to have been turned into stone. There are Fossils, in which the particles of earth have so combined themselves with the original substance, as, without destroying it, to make it stone: to these the term Petrifactions very properly applies.

MAT.-Can you show us the different kinds of Fossils?

MRS. L.-Here is a small shell, which has been so well preserved, perhaps for centuries, beneath the earth, that you would scarcely know it from a shell gathered up but yesterday. Fig. 1. is a specimen of the Ammonite Fossils, not to be found in a recent state. Here is a specimen (Fig. 2.) that will show you several kinds at once. Observe that while a part of the shell (a) remains but little changed from what it originally was, the Iron Stone (b) has filled it, and formed itself, as in a mould, to the shape of the shell; and were the shell gone, it would retain its form and seem to be the shell itself transformed to stone. At (c) in the same figure, we have another kind of Fossil-the shell is gone, and the impression of it only remains on the stone. Fig. 3. is a piece of Fossil Wood: that it is wood your eye will instantly pronounce; but scratch it, weigh it, or crumble it in pieces, and you will find it so impregnated with earth, as to bear far more the character of stone than that of a vegetable substance.

MAT.-I perceive the difference perfectly. This last is the only petrifaction. But now I am curious to know.. how this happens. These substances above the earth would decay altogether. Why do they not so here?

MRS. L.-You know that to decompose an animal or vegetable substance, so as to separate entirely its constituent parts, which is what we call decay, two things are necessary-air and moisture. Exposed to these, organized substances deprived of life are decomposed, the particles are separated and dispersed in new forms, and the substance disappears. Deprived of either of these, the process is different, and the results very various.

To speak first of vegetables. When perfectly dry, hay as a vegetable substance, however much exposed to a dry air, will not further decay, though it has changed its form materially since it was living Grass. When moisture on the contrary is admitted to the same substance without air, as is the case under ground, a very different result ensues. In the formation of Peat, the vegetable matter loses entirely its original form, assumes a dark colour, and a strong bituminous odour. In places.favourable to the accumulation of water and of the peculiar plants of which it is composed, immense quantities of it are formed. In these Peat-bogs, trunks of trees are often found imbedded, partaking of the substance of the surrounding mass, mineralized in different degrees, according as they lie buried deeper, or have remained there longer. This is called Bituminous Wood. In the same situation is formed a different substance, called Moss Fir. It looks like decayed wood; but all its fibres and interstices are filled with resin. It is so highly inflammable, as to be used by the poor for torches as well as fuel. Another kind of bituminized wood is the Bovey Coal. In near approach to this is the beautiful substance we call Jet.

MAT. Do you mean that Jet, used for ornamental purposes, is fossilized Wood?

MRS. L.-Undoubtedly. You must know it too well to need a description. But do you know this substance? MAT. It is very like Jet, but I believe it is Coal.

MRS. L.-It is Cannel Coal, a substance differing from Jet, perhaps, only in containing a greater portion of earthy matter.

ANNE. Is all Coal, then, a vegetable substance?

MRS. L.-I feel no doubt that it is; though some have maintained otherwise. It seems to differ from the above only in containing Spar and Pyrites, which give it a different fracture, smaller grained and uneven. But we will leave the further mention of Coal, till it occurs in its place among the Secondary Strata. On Gg

VOL. VI.