54v CALCULI.

Very striking- but some plates of it which I have seen polistsd, have equal’d mostof the stones of this kind. . It is of a considerably fine, smooth and compact tex-ture, generally os* a very rugged and unequal surface, very deeply wrinkled in theusual manner of Pebbles, and full of large wart-like excrescences. It is generally ofa very compress’d, but orbicular form, and is from two to six inches in diameter. Itis compos’d of a large brownish yellow, or tawny central Nucleus, environ’d with afew, but those generally very broad and thick crusts, some of a greyish black, andothers of a tolerably clear brown, but without the yellowness so constantly mix’d withthe brown in the Nucleus: a circle of paler grey than the rest generally immediatelysurrounds the Nucleus ; after this the brown and darker grey lie alternately, and aregenerally of about the same breadth one with another, and very beautifully terminatedat their edges, not running into one another, as the circular crusts of many of thePebbles do. The outer crust is ever of the brown, and generally is but thin, and thecoat over that is likewise thin and blueish, but much paler colour’d on the protube-rances than in the other parts. This species usually keeps pretty regularly to this ap-pearance, but is sometimes found without a Nucleus, and with the agate-like dispositionof the variegations; and at others is confusedly blended one part with another, andmakes a dirty looking stone, of a mixture of both the colours. It is less transpa-rent than many of the other species. .......

It breaks with an even and glosiy surface j and is very hard, and capable of a sinepolish. ..■ . ■ ' ...

It very freely gives fire with steel.

It will not at all ferment with Aqua Fortis.

It burns to a reddish grey.

Examin’d by the Microscope, the blackest parts appear of a pale .grdy, the grey al-most colourless, and. both made cloudy by a large quantity of very minute earthy Mole-cular ; the brown parts are more pure, of a more equal texture, and more pellucidthan these, the Nucleus is of a smooth-texture, but most opake of all.

■ It is not uncommon in the gravel pits of Hertfordshire, Buckinghamshire, andNorthamptonshire, but is very seldom seen about London.

32 . Calculus nucleo pallide cinereo^cruftis albis virescentibus.Pale-grey center d Pebble , with white and greenifo crujis.

This is a very elegant Pebble, and tho’ of very little variety of colour, is yet ex-treamly beautiful. It is of a moderately fine texture, and very firm and compact : ofa smooth and even surface, very little subject to wrinkles, and not at all to any. otherirregularities. Its figure is almost constantly very round, and not at all flatted, andits. size from one. to three inches in diameter. It is compos’d of a small orbicular cen-tral Nucleus, of a pale ash-colour’d grey, surrounded with a considerable number ofcrusts of two colours, t some of a very pure white,; without even the least admixture ofblueness, so common in the : white crusts of the finer Pebbles; and others of, a palegreyish green, a faint, but not disagreeable colourT a circle of white usually imme-diately surrounds the Nucleus, and often the commissure between them is in a mannerobliterated, and the whole has only the appearance of a grey spot, with a white edge;this is owing to their blending in some degree together, from their having been ap-ply ’d to one another, while both were yet very soft and moist ; but-tho’ this has some-times happen’d, instances are also frequent in which the commissures are distinct,and the edges of both the Nucleus and first crust, perfectly determinate and very beau-tiful ; the crusts that succeed this, are plac’d in a beautiful alternate order, the whitealways the broadest, the greenish the narrower ; sometimes also these are blended in agreat measure with one another, and have undulated edges running thro’ one anotherto the succeeding crust, but more frequently they are very exactin their edges; andin that cafe it is, that the stone is most beautiful, the colours being all of a palehue, and in some measure alike, and regularly parted from each other, making a veryagreeable appearance. The outer crust of this is usually of a paler, or greyer greenthan the rest, and but thin, and the coat that covers it is very thin, and of a verypale blueish ath-cdlour. It will be easily conceiv’d, from what has already been ob-serv’d of the veins of-this stone, so . frequently running into one. another, that it is houncommon thing for them to be found absolutely blended. together, and forming

one

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