452 SYM.PE X I A.

There is no doubt, but that the fluid out of. which this Stone was form’d by meresubsidence, was of a consistence to bear the shells suspended in it while in motion; andthat this mass was form’d by the subsiding of quantities of these, and of stony matter al-most at the same instant of time; for in the closest assemblages of shells, there isstony matter among them, and no part of the purer stony mass is wholly free fromshells, or the remains of them. •

This is brought in great quantities from Purbeck, and us’d in the flat pavementsin London.

8 . Sympexium marmoreum , albido-subcmereum.

TVhitijh grey , marbly Sympexium.

This is a very beautiful and very useful Stone ; it is of a considerably fine, and very firm,even and compact texture, remarkably heavy, of a whitish colour, with a saint admix-ture of a blueish grey, which is sometimes so flight as scarce to alter it from a pure tho’dead white ; and has a remarkable tho’ not strong smell, which is faintish, and in somedegree resembles that of the Florentine Iris Root where fresh broken; it is also some-what of a sparkling appearance, and is not unfrequently ornamented with Dendritæ andother black marks from accidental flaws and cracks; it never contains shells, so far as isyet known ; it is considerably hard, and capable of a tolerable polish ; water pour’d onit does not at all pervade its structure, but oil continually rubb’d over it by degreeschanges its colour to a deep brown.

It gives fire very freely with steel.

It will not at all ferment with Aqua Fortis.

It burns to a pure white.

Examin’d by the Microscope, it appears of a very close and even texture, considera-bly bright, and full of shining particles, and in thin pieces is a little transparent; andseems compos’d of crystalline matter, highly fated with a foul whitish earth.

It is not found in England, but is extreamly common in large strata in all the Easternnations, and even in Italy, and some parts of Germany.

It is in great use with our artificers for setting a fine edge on their tools, and is com-monly known among them, by the name of the Turkey Stone; it is not unfrequentlybrought to us in thin flat pieces lying on the stratum which has been next beneath it inthe earth, and this being usually a black one, has a very pretty appearance.

9 . Sympexium filiceum , albido-flavescens.

Yellowish white , flinty Sympexium.

This and the two following, approach to the texture of the common Flints in ap-pearance, and the next described has really much of their nature.

This is a very fine, smooth and glofly Stone, of an extreamly firm, close and com-pact texture, very heavy, and of a somewhat brownish, or yellowish white ; it neverthat I have yet observed contains shells : it is extreamly hard. Water poured on it,enters not at all its texture; but notwithstanding its flinty appearance, it yields fire butdifficultly with steel.

It makes a very great and very lasting effervescence with Aqua Fortis.

It burns to a greyish or blueish white.

Examin’d by the Microscope, it looses its flinty appearance, and is seen to be a plainsemi-pellucid, yellowish white crystalline Spar, a quantity of which concreting alone,and without all heterogene admixtures in the once universal fluid, seems to have form’dthis Stone.

It is common in Northamptonshire, Leicestershire, and many other parts of England,where among other strata of a coarser kind, it is known by the name of Ragstoneand Limestone ; it never constitutes strata of any great thickness, but generally lk sdeep among the Arenaria; it is generally very free from fissures, but where there areany, their sides are always encrusted with a crystalline Spar.

It is us’d in the pavements of streets, but is too rough and rugged, and too hard to cutto come into use in building; in many places they burn it into lime.

10 . Sympexium