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CONVERSATIONS ON GEOLOGY.
Rocks their Characters and Classification.
MATILDA. I am impatient to proceed with our study, and to be informed what is the first known covering of the unknown Nucleus we were speaking of in our last conversation; for I see you are beginning at the centre and proceeding upwards. Is that the usual course?
MRS. L.-Generally, but not always. In the excellent Geological treatise of Messrs. Conybeare and Phillips, with which you will hereafter be acquainted, the contrary method is pursued, and the various substances of which the earth is composed are traced from the surface downward. I preferred the other course as calculated to convey a clearer idea of the whole-the more internal substances seem to be the more simple, and, strictly speaking, the first, since they are the foundation and supe port of all the rest, and, as we believe, the earliest formation.
ANNE. You now, Mama, speak of Rocks, as distinct I suppose from Minerals and Earths-but I should like to clear my ideas upon the subject by having the term exactly defined.
MRS. L. That is not easy. The compilers of systems, as Macculloch observes, have been divided respecting the substances to which the title of Rocks should be applied. I have told you that Minerals are distinguished from Rocks by not forming large masses, but being dispersed among others: keeping this difference in mind, together with the popular meaning of the term Rock, which is very well understood, I do not think you can be much misled in the use of the word. "The characters by which Rocks are distinguished from each other, are their
Structure, Texture, Fracture, Hardness, Frangibility, Lustre, Transparency, Action of Acids, Specific Gravity, and Colour"-characters that I shall briefly explain to you now, to prevent interruption hereafter. Structure is either External or Internal. The External Structure of a Rock is the form of the mass in which it is found, whether in large irregular masses, as the Rocks of Granite we shall presently describe, or in Strata, Nodules, Veins, &c. explained to you in a former conversation. The Internal Structure of a Rock is described in terms I must endeavour in some way to explain, though I am aware of difficulty in defining them. A Rock is said to be of a Laminar, Lamellar, Foliated, or Schistose Structure, when it is disposed to divide, either by force or the action of the weather, into plates-or when it has the appearance of being made up of parallel plates without actually splitting. These plates may be either straight or curved, or in various ways contorted-they may be of the thickness of yards or as thin as paper-the terms will still apply. Slate will readily occur to you as an example of this structure. Granite is so also-but on so large a scale, that you could only observe it by examining the Rocks as they stand. There is some small difference in the three last terms, but to attempt to distinguish them now would only embarras you. The term Laminar is made to include them all.
The Prismatic Structure is when the Rock assumes the form of Prisms or Columns, composed of lines and angles, that give it almost an appearance of architectural uniformity, or as if art had come in with her tools to shape the productions of nature. The famous Basaltic Rocks of the Giant's Causeway, of which you have doubtless seen drawings, are an example of this on the large scale. The forms and shapes are endlessly variable, but you may, I think, form a general idea of the structure. The action of air or other circumstances will frequently wear away the edges and angles of rocks of this form, so as to make them assume the appearance of a Spheroidal Struc
ture-a term you may understand as including all that are not either Laminar or Prismatic.
The Structure is said to be Veined, when veins of the same substance, or slightly differing from it in hardness, colour, &c. run through the mass: Cavernous, or Cancellated, when full of holes or channels: Amygdaloidal, when these cavities are filled with mineral substances foreign to the Rock: Aggregate, when different kinds of Rocks previously broken into fragments of various sizes, are reunited into one solid mass.
The Texture of Rocks is nearly the same thing as their Structure, and yet one word may be applied where the other cannot. I can tell the Texture of Granite by holding a small piece of it in my hand, I cannot perceive its Structure without seeing the rock itself: but most of the above terms, as well as those that follow, will apply both to the structure and texture.
A substance is called Granular or Crystalline, when grains of the same or different minerals are so closely aggregated as to destroy each other's form, so that though you clearly perceive the confused mixture of separate pieces, you cannot determine the shape of any.
Porphyritic, is when distinct though minute crystals, of one or more substances, are imbedded in another compact or granular substance: Fibrous when the appearance is stringy, or the crystals so long as to have the form of threads; the term indeed explains itself; the fibres may be parallel, or curved, or crossing and interlacing each other, or they may radiate as from a common centre. The term Scaly also explains itself; but we shall perceive that if the scales become very minute it will have a Granular Texture, if large a Laminal: Compact implies the absence of all the above forms; a substance in which there is no appearance of separate parts.
Another distinguishing character of Rocks is the Fracture, that is, the appearance of the surface when newly broken. Some substances will break even, others un