CALCULI.

539

30 . Calculus nucleo , nigricante , crujlis nigris albidis .

Black center d Pebble , with black and white crujls .

This is a very beautiful species, tho’ it never has any farther variety than fromthese two plain colours. It is of an extreamly fine, close and compact texture, of avery rugged and unequal surface, generally full of deep wrinkles, and of largeWart-like protuberances, often of the bigness of a horse bean. It is usually of acompress’d form, approaching to round, and frequently of four or five inches in diame-ter. It is compos’d of a moderately large central Nucleus, of a fine glossy black,surrounded by a vast number of alternate crusts, of a beautiful blueifh white, or pearlcolour, and of the fame colour and substance with the Nucleus; a narrow circleof pearl colour always surrounds the Nucleus, and alternate circles or crusts of black,and of that make up the rest of the stone. The white, however, are all very narrow,the others broad, and the outer one being ever of the black, and broader than therest, makes upon the whole the black look like the general basis of the stone, and thewhite no more than a flight variegation. The coat which surrounds this outer crust,and covers the whole, is generally very thick, and of a coarse chalky looking opakewhite substance. There are at times found instances of this species, like the rest wantingthe Nucleus, and having its white matter diffus’d in irregularly difpOs’d, but all con-centric lines, as in the Agates, this having been the effect of the diffusion of it, bythe motion of the ambient fluid at its one general coalescence into a mass ,• all these be-ing, not as the Pebble would have been in its more natural state, the effect of thesucceffive application of fresh quantities of matter in form of crusts, but form’d bythe concretion of all the matter that forms them at one instant of time, and into onemass. Sometimes also we find a pale Or darker grey stone, in a white thick crust, notwell reducible to any other species, but a manifest variety of this from a more irregu-lar mixture of its two differently colour’d substances; which in this cafe appear tothe naked eye to constitute one simple substance, of a pales or darker grey ; but ex-amin’d more attentively, and with the help of glasses, this is generally discover’d to bemade up of numberless Moleculæ of each colour, often running into. one another, andsometimes almost perfectly blended together. It is of more transparency than many ofthe other species.

It breaks with a very fine, even and glofly surface, and is extreamly hard, and ca-pable of an elegant polish.

It very freely gives fire with steel.

It does not at all ferment with Aqua Fortis.

It burns to a dusky grey.

Examin’d by the Microscope, it appears of an extreamly elegant structure, and con-siderable transparence in both its colours; the white has much of the cast of the blueifhAgates, and the black view’d in the thin flakes proper for this fort of examination,loses so much of its colour, as to become of a pals grey, and very beautifully pellucid;but its debasing earth is more visible in clouds or small Moleculæ than in the white.

It is not unfrequent on the Sussex and Yorkshire shores, but it has never yet hap-pened to me to find it in digging. This and some other stones of different colours,and much of the texture of Flint, found in the fame places, may make the youngstudent in these things start at my declaring that Flint is but one species; but it is tobe consider’d, That these differ from Flint according to certain and invariable rules andcharacters, to be fsen under the general character of each Class; and that this is agenuine Pebble, and those others are Agates, such as we have in England.

I have more than once met with this species, wrought by our lapidaries into sealsand other toys. It has much of the look of an oriental stone, and generally is ho-nour’d with the name of an Onyx or Sardonyx, so careful • and determinate are thosegentlemen in these things.

31 . Calculus nucleo Jlavo-fusco , zonis fuscis & cinereo-nigrescentibus.Tellowijlo brown center d Pebble, with brown and greyijh black crujls .

This is a Pebble, which is, if closely examin’d; another very beautiful one, tho’with very little variety of colour: at a distance indeed, and in its rough state it is not

very

>