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even-that is, with elevations and depressions on the surface-others will break Conchoidal-that is, with one surface concave, the other convex-a succession of Conchoidal figures will produce an undulated surface--sometimes they will break Splintery, sometimes Hacklywhich we should describe as the points or edges of splinters or fibres broken across. You will perceive, that in many cases the Fracture will be the same thing as the Texture.
The remaining character of Rocks are Hardness-a term entirely comparative, and therefore the degrees are not describable; it may be best judged of by the resistance offered to the point of a knife. Frangibility, the degree of resistance offered to a blow-some stone will break with a slight stroke-others will scarcely break by any force. The frangibility of a Rock will often depend on the quantity of water it contains-when taken from the interior of a mass, it will sometimes be extremely brittle, but become tough by a few days' exposure. Lustre is also comparative, and may be best estimated by the substance it most resembles in lustre: thus it may be called Plumbaginous, Silky, Resinous, Vitreous, Flinty, or Waxy, according as its lustre resembles that of Lead, Glass, &c. Transparency is a word you well understand, Rock is never transparent; but a thin splinter of it may transmit light at the edges, and is then called Translucent. The Action of Acid upon substances is often a good method of distinguishing them; but this we cannot enter upon at present. Specific Gravity is another character, but not generally of use. Colour is not much to be depended on, being very variable, and the tints too indeterminate to be conveyed in words. Very frequently Mineral substances receive their colour from the various modifications of Iron intermixed with them. Still, though we cannot always well describe the Colour of Rocks, it is not to be disregarded, and our plates may assist you. Do you think you now understand the characters, or at least VOL. V.
their terms, by which Rocks are distinguished one from another?
ANNE. I think so here is a piece of Porphyry (Fig. 1. Plate 3.,) let me try and describe it. The Texture I should say is Granular, but that beside the confused intermixture of different substances that form the base, there are distinct specks of white, as of crystals separate from the rest-this you call the Porphyritic Structure, and it is different from this piece of Granite, which is indistinctly grained throughout. The Fracture appears to me uneven, as I can by no means break it into a very smooth surface. It is very hard, for my knife makes no impression on it. As to the Frangibility, I broke the Granite with more ease than the Porphyry-but that, as you observed, might be accidental-it cost me some trouble to break either, therefore, I suppose, they are not remarkably Frangible. It is not in the least degree Transparent, so perhaps I may call it Opaque, though you did not use the word. As to the Specific Gravity, I have nothing by which to determine it, and you said it was not of much consequence-the Colour is reddish with white specks. As to the Structure, it is impossible for me to discover it by this small piece, so far removed from its native bed. Of the Action of Acids you have told me nothing-so that after all, if I did not know this to be a piece of Porphyry, I have but few tests by which to discover it.
MRS. L.-I expected you would come to that conclusion; but now you have seen what you know to be a piece of Porphyry, and have examined and described it thus, do you not think you would know it again if you saw it; and if it should happen to be green instead of red, should you not still be able at least to conjecture what it is?
ANNE. I think I might.
MRS. L.-That is all you must at present expect. The examination of specimens, with attention to their distinctive characters, will by degrees make you familiar
with them. When two substances occur that nearly resemble each other, you must try to find the easiest test by which to know them apart: as in this instance you have observed one marked distinction between Granite and Porphyry, else not unlike each other in appearance. This is not the place for describing either; we shall reeur to them; I merely wished now to ascertain if you understand our terms sufficiently to appreciate their meaning in future descriptions. You took no notice of the Lustre.
ANNE. Because it is not all alike-some part shines, some does not.
MRS. L.-Exactly so-because Porphyry is composed of several substances, some of which are brilliant, others are not-therefore it would be difficult to give a word that might convey the degree of lustre on the whole: perhaps we might call it of a Flinty Lustre, if it were necessary to give it a name. But you see how little accurate these comparative words can be-you must expect your knowledge from experience and observation, rather than from words. In studying the composition of the earth's surface, some division or classification of subjects is necessary; but on this Geologists are not agreed. It is not of great importance which mode of division we adopt; but I should wish to prevent your being at a loss, should you take up a work that adopts a different mode, therefore must take some notice of these differences. The more usual division has been into three series of Rocks: the Primitive or Primary, the Transition, and the Secondary. This division arose from the supposition that the first Formation or Series were coeval with the creation of the world; that the second had resulted from the Deluge, or some great catastrophe occurring since the creation; while the third owed their formation to the partial revolutions and gradual changes taking place on the surface. If this were so, it would be the most natural division, and therefore the best; but that is by no means certain. If, therefore, I adopt it now, it is because it seems to me the easiest to understand. Other Geologists make but two divisions of
the three-including the Transition Class in the Primary; and as there is really no very definite line to be drawn between them, there is no reason against this arrangement: while others again call Secondary what we call Transition, and our Secondary they call the Floetz, or Flat Formation.
MATILDA.-Will not this difference cause us some
MRS. L.-I shall endeavour to prevent it as we proceed. As general remarks on these three series, each of which we are about to describe minutely, you may bear in mind that the Primary, supposed the most ancient, are generally found in huge masses or blocks, not regularly stratified, and in a vertical or upright position. They are mostly hard and durable, alike in their tecture throughout, and composed of two or three ingredients blended together: the texture is generally crystalline, and they constitute the loftiest mountains. The Transition series, supposed to be next in antiquity, are less lofty than the former; they often present a slaty texture; seem to have been deposited in Strata or Layers, and these are seldom either vertical or horizontal, but inclined to the horizon, as I described to you in Plate 1. Fig. 3. The Secondary Rocks are nearly or altogether horizontal in their position-seem to be of more recent formation-are softer in texture and consequently more liable to change and decay. "These different series are tolerably regularly arranged in regard to each other. The Primary Rocks form the bases upon which the others rest; the Transition are immediately recumbent upon these; and these are succeeded by the varieties of Secondary Rocks; from the wreck and decay of which is formed the Alluvium, or what we commonly call the soil: besides this Alluvial matter, there is the Volcanic, the immediate product of Volcanos, and the Trap, or Overlying Rocks. Of all these we shall speak hereafter; and together they compose the whole of what we have called the earth's surface: that is, of its known substance the shell that surrounds the unknown
Nucleus. Can you, from Fig. 3., point out to me the different series?
ANNE.-I will try-a a, I observe, stand in a vertical position, rear their heads above the rest, are of a massive form without any describable shape-I cannot see their base, but from the firmness and largeness of their structure and position, I can imagine them to be the foundation and support of all the rest: these therefore I suppose may be the Primary or Primitive Rocks. From their oblique position on the sides of these, and from the succession of beds or Strata of different colours, I conclude bb bb to be the Transition Class -while c c c from the more horizontal position, doubtless describe the Secondary. I observe you have drawn over them a surface (d d) of something that I suppose to be Alluvium or soil, clothed in the verdure that hides these secrets from our view.
MRS. L.-Yes, because this is the more common case-but you may observe that it is not necessarily so there are spots where these Formations lie exposed, In our next conversation I shall describe to you the characters, supposed origin, uses, &c., of the first or Primitive Formation.
HENRY. I was much amused the other day, by
reading an account of an attempt made to set aside