matter, has this faculty of giving it a stoney hardness; of which some actual examples were shewn to me by an ingenious and learned member of the Royal Society *.

VOL. X.

I

The

* Edward King, Esq. Since I saw the petrifications above mentioned, Mr. King has given a very curious account of them, and some others of the same class, in the Philosophical Transactions for the year 1779, which the philosophical reader cannot fail to peruse with pleasure. When we are once possessed of a true principle, it is easy to trace it in a multitude of examples which were not attended to before, though obvious enough in themselves. The petrifying power of a solution of iron upon sand, gravel, and clay, is by no means an uncommon phænomenon; and I won. der it did not occur to me before Mr. King pointed it out. Upon the beach at Harwich, near the water's edge, many tons weight of this sort of petrifaction are to be seen. smiths throw out their iron slags and cinders from the forge, and lay them in heaps upon the shore, where the sea washes up sand and pebbles upon the iron; and the particles of iron being partly dissolved by the same cause, the gravel, sand and iron unite together into a compound mass of a ferrugineous colour, and the bulk of the iron slags is continually increased; till by degrees, when the intervals between several of these masses are filled up, they concrete together till they form an indissoluble rock of beach and iron.

The

I have lately been informed of another fact of the same kind, by a friend of mine, who is concerned in an iron foundery for cannon in Wales. He tells me he has frequently seen fragments of iron guns with part of the mold of clay adhering to them, in a state of perfect petrifaction: Also sand,

become

114

bles are

The black, white, and dove-coloured marcommonly known, and are chiefly brought from Italy. Some of the most valuable sorts are the granite and porphyry : the granite is so called, because it appears like small grains, mixt with another colour which forms the ground. Porphyry is nearly allied to the granite, and has its name from the purple colour of its ground. It is the hardest of all marble; yet antique vases were formed of it, and figured with great art. Its present hardness may be partly owing to its

age.

Slates of all kinds are distinguished by their splitting with a regular grain, and sometimes to the utmost degree of thinness. The black

slate,

become so hard by its contact with the iron, that it could not be broken off from it but by an hammer.

On the coast of Kent, I have found masses of a stoney hardness, with multitudes of recent shells fixed in them; for which I could never account, till I became acquainted with this power of iron in petrifying other fossil substances.

These facts are not only curious in themselves, but they lead us to a new principle which is of great value, and will teach us to compose cements for building, more durable than any hitherto invented, and better able to stand all the attacks of the weather, which will rather strengthen than impair them. We are indebted to the ingenious Mr. King for this hint, in his paper upon the subject; and there are many other observations both learned and useful in the same paper.

slate, when polished, is used for writing. The blue and brown slate is used for covering houses. The method they have of splitting it is this: they expose it in blocks to be well wetted and soaked with the rain; and when the frost comes upon it afterwards, it rarefies the water, and thereby cracks and opens all the joints of the stone; so that when the frost has done its work, it lies in loose flakes or shivers. Thus the elements perform with ease what no manual art could possibly accomplish.

There is a sort of slate in Yorkshire, out of which they extract the salt called alum, by preparing the stone in a particular manner with urine.

The joints of slate, particularly the Irish slate, which is used in medicine, are sometimes figured over with an exsudation which disposes itself in the form of trees, or branches, and composes the fossil which is called dendrites.

The talc or selenite is found of many dif ferent figures; sometimes in rhomboids, sometimes in hexagonal columns: they are generally found in clay, and seem to have been formed out of it, as there are sprigs of clay, not rightly assimilated, found in the middle

of them. This substance is generally transparent, sometimes as fine as glass, and will split into very thin flakes. Windows were formerly glazed with the large rhomboidal selenites split; and thence was derived the form of our ancient diamond panes in glass windows. The selenite burns to a very fine calx, and is used abroad for forming a kind of plaster of Paris.

The gypfum is nearly related to the talcs and selenites*; it is called alabaster, and vulgarly plaster. Sometimes it is hard, white, and opaque, like statuary marble. There is another sort, which is composed of fibres, flakes, or plates, and is transparent. The gypsums are found so frequently in marle, that it seems as if marle were a folution of gypsum, or gypsum a condensation of marle whence it is well observed by a late author, that gypsum seems to have the same relation to marle, as flint has to chalk. The alabaster, when burnt, makes the finest sort of lime, which we call plaster of Paris.

The asbestos is so called from its enduring the fire. It appears like hairs or threads, and is capable of being spun and woven into cloth.

* Linnæus calls the selenite gypsum crystallisatum, figurâ rhomboidali. Syst. Nat.

cloth. It is supposed that the ancients, who burned their dead, wrapped them up sometimes in a cloth made of this stone, and fo preserved their ashes entire for the urn.

The waren-vein is found in nodules of clay-stone, quartered out into tali by partitions of a sparry substance, of the colour of bees-wax, which partitions pass through the whole mass of the stone, and penetrate at the same time all such extraneous bodies as happened to be deposited in the matter of the nodule at its formation. The stoney matter of the nodule is of different colours, but in general the partitions or scams which divide it are sparry. The same matter is found in the pipes or worm-holes of petrified wood; and sometimes it is disposed, in the form of stars, between the cavities of the stone.

The atites, or eagle-stone, is a sort of pebble with a cavity, and in that cavity a nucleus which is loose, and rattles within the stone when shaken.

The enhydros is another body of the same kind; but, instead of a solid nucleus, it contains water, The Earl of Bute has a stone in a ring, not bigger than a pea, which has a cavity containing a drop of water, which may be seen to fluctuate through the trans