96

CONVERSATIONS ON GEOLOGY.

CONVERSATION XIV.

Gypsum-Alabaster-Plaister of Paris-Fluor Spar-Salt-Coal.

MAT. I think, in our last conversation, we had done with the Red Sandstone, had we not?

MRS. L.-With the Sandstone itself, but not with its contents-subjects of no small interest. I believe I mentioned to you that of these, Gypsum, Rock Salt, and Coal are the most important.

MAT.-Salt and Coal are familiar acquaintance, but I have no idea what Gypsum is.

MRS. L.-Not quite a stranger, under other names. Gypsum, or Sulphate of Lime, (Lime and Sulphur) occurs in great abundance in many parts of the Red Sandstone Strata. It is known also under the name of Plaister-stone, Selenite and Alabaster. When the common Gypsum is heated to redness and loses the water it contains, it falls to powder; which ground fine, is what we call Plaister of Paris. Near Tutbury in Staffordshire, and near Nottingham, it is found in blocks and veins; and lately a variety new in England has been found, called Anhydrite. These minerals constitute valuable materials for the ornamental manufactures of Derbyshire.

ANNE. I had no idea we were speaking of the beautiful Derbyshire Spar.

MRS. L.-It is not so exactly. The substance of which those beautiful vases and other articles are usually made, is confined to one mine-it is called Fluor, and differs from Gypsum in that the Lime is mixed with Fluoric Acid, instead of Sulphuric Acid, which forms the yet more beautiful substance we call Alabaster, which is a variety of Gypsum. Gypsum is soft and may

be scratched with the nail, which sufficiently distinguishes it from Marble.

MAT.-I should like to see some specimens.

MRS. L.-I will show you several-Fig. 1. is a specimen of common Gypsum.

MAT.-I should take it for marble.

MRS. L.-Not if you feel it. It is soft when first taken from the ground; and when hard and dry, becomes powdery in your fingers. A pressure of your nail will easily scratch it. These are not the characters of Marble. Fig. 2. is another specimen of Gypsum.

MAT. This is beautiful-it has a streaky, brilliant, and satin-like appearance, quite unlike any thing else I have seen.

MRS. L.-Fig. 3. is Selenite. It is a transparent, crystallized Gypsum, and seems to differ from the others in containing more water.

MAT. This indeed I should have taken for Micaand cannot now see any difference.

MRS. L.-Try to slit a piece off. The Mica, you remember, slits into thin plates, that will bend without breaking. The Selenite crumbles into dust when broken. Fig. 4. is a specimen of Fluor crystallised in cubes. It forms some of the most beautiful of our Spars; arranging itself into enormous crystals, transparent as glass, and of the most beautiful colours. You cannot look into a cabinet or museum without seeing numbers of them, for they are very common. Fig. 5. is one that from its near resemblance to the Emerald is called False Emerald. I need not show you the Foliated Fluor which is the Derbyshire Spar, less transparent and not formed into Crystals. Gypsum is found in cavities of the Sandstone, sometimes forming a considerable extent of country, and rising into low hills. "In this manner it is found not only in England, but in different parts of the continent of Europe. It is also a conspicuous member of that peculiar district round Paris, which is supposed to have been deposited under the waters of an inland lake.

Organic remains of great interest are found in the Gypsum of the Paris fresh-water deposit. These are the bones of various terrestrial animals of extinct species and generą; besides which it also contains shells and remains of fish and vegetables. But I believe the Gypsum of Paris is considered to have been formed later; as in that which is found among the Red Stone, of which we are now speaking, there are no organic remains. Our next subject is Salt, of which I suppose you will not desire a specimen.

MAT.-No-but I should like to know more of the manner in which it is deposited in the earth.

MRS. L." In the county of Cheshire the Red Sandstone contains immense beds of Common Salt, most abundant in the valley of the Weaver; and near Middlewich, Northwich, and Nantwich, it is accompanied by Gypsum. The first stratum was discovered about one hundred and fifty years ago, in searching for coal. It begins about thirty yards below the surface and is twenty-five yards thick. Below this, and separated from it by ten or twelve yards of hard Clay, is another bed of Salt, the extent of which is unknown; in many places of extreme purity, in others tinged with Oxyde of Iron and Clay. This pit is at Northwich; and at other places there are very abundant brine springs. From these salt. is procured by evaporating the water. A most remarkable circumstance in the Northwich mine is the arrangement of the Salt, giving rise to an appearance something like a mosaic roof and pavement, where it has been horizontally cut. The Salt is compact, but it is arranged in round masses, five or six feet in diameter, not truly spherical, but each one compressed by those that surround it, so as to have the shape of an irregular polyhædron. The Wernerians regard the Salt as having merely crystallised here after being dissolved in water; the Huttonians consider the water to have been evaporized by heat. The large pit at Northwich presents a very singular spectacle when duly illuminated; it is a circle of nearly