HOMOCHROA.

1

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4 . Homochroum subcœruleum .

Blueijh Homochroum.

This is a more impure and less elegant Pebble than any of the former; it is of amoderately compact, smooth and close texture, of a rude, irregular and much wrinkledsurface, and very uncertain in shape and figure, being sometimes roundish, sometimesoblong, but oftner of an unequal and protuberant shape ; its general size is abouttwo inches in diameter, but is often found much exceeding this, or much smaller • itis not often found so pure and simply colour’d as the former species, but when it is, isof a somewhat dusky lead colour, or impure blue ; this may be esteem’d its proper ap-pearance and most perfect state, but this it is but seldom found in, it being tho’ when itssubstance is thus perfectly blended of one simple colour, yet really of two, a dusky blue,and very opake white, the most usual state we find it in is with these colours a littlegot loose from one another: it is usually of different degrees of blue in its differentparts, in some deep, and in others approaching to white, but is very frequently alsoseen with the two colours more separated, the whitish appearing almost disengaged fromthe blue, and making a thousand oddly figur’d clouds, spots and short lines on it; thisis the effect of the disunion of the two substances which go to compose the proper co-lour of this species, and in this state it is most ordinarily found ; it is, however, sub-ject to no other adulteration, or mixture of colour, nor have I ever seen a single in-stance of the white in this making any thing like regular lines, veins, or crusts, or seem-ing as if an additional matter thrown into the composition of another stone, butever as a true and genuine part of it, in some degree loose and disingag’d from therest, and ever rendering the stone the worse by being so; this is generally perfectlysolid to its center, free from cavities, and from those imperceptible cracks to which theformer species and many others are so subject. Its crust, or outer coat is very variousin colour, ordinarily it is white and chalk-like in appearance, but sometimes it is of ab'lueish brown, and sometimes very thick and yellow; the stone is less pellucid thanmost of the other plain Pebbles, it breaks however with a smooth and even surface.

It is considerably hard, and capable of a good polish.

It very freely gives fire with steel.

It will not at all ferment with Aqua Fortis.

It burns to a whitish grey.

Examin’d by the Microscope, it appears bright and glosiy, but very clouded andopake, and most so in the whitest parts, and least of all in those blue ones, from whichone may fee the white has disingag’d itself, and in some measure got out; it is indeed avery general observation, that none of all the adulterating earths render the stone soopake as the white.

It is very common in all our gravel pits about London, and elsewhere.

5 . Homochroum virescens.

Greenijh Homochroum.

This is a very foul and opake Pebble, but for the singularity of its colour is very re-markable, and generally strikes the eye wherever one happens to meet with it. It is ofa moderately compact and close texture, but less fine than many of the common species ;of a pretty smooth and even surface, having generally but few, and those but flightWrinkles, and seldom any prominencies or cavities about it. It is generally of an orbi-cular or oblong figure, and varies in size from half an inch to two inches in diameter,beyond which it is seldom found. It is ever absolutely free from all foreign admixtureof colour, and has neither spot,' cloud, or vein of any, nor the least variation either indifferent stones, or the different parts of the same specimen, except in the degree of tinge.Its colour is a deep dusky, and somewhat blueish green. It has more or less of the bluecast in the different specimens, and is often rather blueish than green, tho’ sometimes ofa pure jasper colour’d green, and without all sign of blue in it; it is but of a coarse look,and is plainly extreamly loaded with earth; its coat is generally thick and white, butsometimes of a dusky blueish grey, and as thin as usually in the other Pebbles; itis less transparent in general than most others, but breaks with a sine, even, andsomewhat bright surface and is very hard, and capable of an elegant polish.

It