538 CALCULI.

mass already form’d, is ever form’d by one simple concretion of the whole mass at oneinstant; and the disposition of the variegations, which are in these veins, and not crusts,is ever the same with that of the Agates, and is owing to the fame cause, the meremotion of the fluid it was originally found in. The stone in all parts is but coarseand opake, but is considerably hard, and takes a better polish than from its externalfigure one wou’d be led to expect.

It freely gives fire with steel.

It does not at all ferment with Aqua Fords.

It burns to a dusky greyish white.

Examin’d by the Microscope, the grey parts appear of the curdled structure socommon to the Pebbles, the earthy matter in it being visibly difpers’d in small clustersof disunited Moleculæ, the other colours are more loaded with earth, and Const"quently more opake.

It is found in great plenty on the Yorkshire shores, and sometimes in our gravelpits.

29 . Calculus nucleo pallide cinereo , cruflis rubris , purpur ascentibusy

sub camlets fufcis.

Pale-grey center d Pebble , with red, purple , blueijh , and browti crufls.

This is a Pebble of more variety of colour than most species, but it wants the fineand pure texture some of them have, to make it shew them to an advantage. It is ofa moderately smooth and even texture, and very firm and compact, of a surface muchfurrow’d with deep wrinkles, but free from any other inequalities, generally of a prettyregular orbicular figure, and commonly of about four inches in diameter. It is com-pos’d of a small central Nucleus, of a pale grey, or whitish ash colour, surrounded witha very remarkable number of thin or narrow crusts, of great variety of colouring;some of them are of a strong red, others of a fair and good purple, others of a greenishblue, and others of a fine glofly brown; these last are of the finest texture, and mostbrightness and transparence of any part of the stone; the rest, tho’ of very agreeablecolours, have a coarseness and dusky look, which greatly takes off from their beauty.The Nucleus is generally immediately surrounded by a somewhat broad circle of red,very much resembling the matter of the ordinary red Pebble, but more foul’d with earth,and consequently less bright and clear; next that is generally a circle of brown; as for therest of the crusts, they are difpos’d without the least regularity, but make a very pleasingand elegant variegation; the outer crust is generally of a blueisli grey, and thicker orbroader than the rest; the coat which surrounds this is usually very thin and whitish.It is a very elegant stone in this state, but is liable to the same varieties with the rest ofthe Pebble Genus, and when it wants the Nucleus, and has its variegations difpos’d inthe manner of the Agates, which is very frequently the cafe, makes often a yet moreelegant figure than when they are rang’d in regular circles ; in its more usual andnatural state it has less brightness, and when cut into plates, less transparence than mostof this Genus.

It breaks with a smooth flinty surface, and is considerably hard, and capable of anequal, tho’ not perfect polish.

It very freely gives fire with steel.

It will not at all ferment with Aqua Fortis.

It burns to a mottled greyish reddish, and brownish mass.

Examin’d by the Microscope, the different colours are all, except the brown, ofpretty much the fame degree of opacity, and appear highly fated with earth; thebrown has some transparence, and shews its earth in form of disjunct Moleculæ, gi-ving a misty look to the mass.

It is a very rare species; I have once or twice met with it about Berkhamstead, inHertfordshire, and in no other place that I can remember; if more common, it wou’dbe worth the notice of our lapidaries; its great variety, wou’d make amends for itsfoulness.

30. Calcuha