CALCULI.

537

27 . Calculus nucleo rubenti , crustis purpureis pallide flavis.

Red center d Pebble , with purple and pale yellow crusts.

This also is an extreamly beautiful and elegant stone; it is of a line, smooth and eventexture, very compact, firm, and close, and of a remarkably rugged surface, full ofinequalities, and tho’ mark’d with but few, yet those very deep wrinkles ; it is gene-rally of a very rude, uncertain, and irregular shape, seldom much approaching to round,or scarce ever at all compress’d ; its size is various, some specimens having been foundof four inches diameter, tho’ the more frequent are of less than an inch : it is com-pos’d of a large, and generally irregularly shap’d central Nucleus, of a strong, deep red,moderately bright and glossy, but with little transparence, surrounded by two or three,soldom more crusts, some of a fine pale yellow, and others 6f a deep blackish hue, asthey lie in the stone, but held up to the light in thin pieces, tolerably transparent, andof a fine deep purple ; all parts of the stone are bright and glosiy, but the yellow veinsare the most transparent, a yellow crust generally immediately surrounds the Nucleus,and what follow that are laid in alternate order, and generally very broad or thick; ayellow one is also generally the external crust, and cover’d by the common coat, whichis usually very thick, and of a yellowish colour on the surface, but very white within,and seeming as if made up of chalk ; this is the usual appearance of the stone, but it isliable to all the varieties of the other Pebbles, and often is found not ill representing avery elegant German Agate.

It is moderately transparent when cut into plates, and is very hard, and capable ofan elegant polish. '

It very freely gives fire with steel.

It makes not the least effervescence with Aqua Fortis'.

It burns to a mix’d reddish brown.

Examin’d by the Microscope, it appears very different in purity and lustre in its dif-ferent parts ; its Nucleus is of a good colour, but very opake, its yellow curdled in themanner usual with Pebbles; and its purple has so much of the black hue of the Pram-nion and Morion of the antients when look’d at in the mass, and so much of theglowing purple of the fame stone when cut into plates and view’d against a good light,that there is no room to doubt its being very much of the nature of that substance.

It is a very scarce species, a few specimens of it have been found on the shores ofSustex, and sometimes it is found in the gravel pits about Oxford ; were it to be hadin any plenty, it would be a very fine stone for our lapidaries.

28 . Calculus nucleo stavo> crujlis cinereis & pallide rubentibus.Yellow center d Pebble , with grey and pale red crusts.

This tho’ less beautiful than some of the former, is a very singular and remarkablespecies; it is of a moderately fine and compact texture, of a very smooth and even sur-face ; subject to no inequalities, and with a great number indeed of wrinkles, but all flightand superficial; it is generally of a very regularly orbicular form, seldom much fat-ted, and from one to six inches in diameter; it is compos’d of a dusky yellow Nu-cleus, surrounded by many thin crusts of a pale red, plac’d alternately with broaderand thicker, of a pale alh-colour, or diluted blueisti grey; all its colours, however,are faint, and dead, and its general texture .much debas’d with earth, and butcoarse ; its Nucleus is generally small, the bigness of a hazel nut sometimes serv-ing as a basis for a stone of the largest diameter ; it is of an opake and curdled look,even visibly so to the naked eye; the grey is much like the colour of some of thepaler Flints, but more debas’d by earth, and consequently more opake; and the flesh-colour is evidently a red not made pale by an admixture of white, but of the greyof the other part of the stone; the grey generally makes the external crust, which ispretty broad and thick, and surrounded by the outer coat, which is ever thin, andof a pale blueish grey; this is the usual arid indeed the natural appearance of thestone, but it varies at times like the rest from the different state and admixture of itsdifferent substances, and frequently wants its Nucleus; in this and all the other spe-cies where the Nucleus is wanting, the variegating matter is never disposed in crusts,but instead of the leisurely application of fresh matter at different times to a smaller

Vol. I. 6 X mass