5 i8 CALCULI.

also ; this being already detach’d from the Nucleus, but cohering firmly to its in-vestient crust behind, can therefore shrink only backwards, that is from the Nucleus,and toward its covering; the plain consequence of this is, the leaving a much largerspace for the Nucleus, and giving it room to rattle as we find it; but' this is not all;the water departing from off" the earth at the end of the universal deluge, must havehad great power, and certainly has in reality done many things, tho’ not all thatDr. Woodward has attributed to it. The reader will allow me, I flatter myself, thatsupposing, as must naturally have been the cafe, the Pebbles, &c. to have been forrn’din it before that time, nothing could be so subject to its force, nor any bodies roll’dand tofs’d about so violently by it as the smaller Nodules, and of these none more thanthe hard and roundish Pebbles : of this we have indeed many undeniable evidences,among which none is greater than this, tho’ some are more evident, as the smoothingthe surfaces of what we call the Pudden stones, the Cementæ before describ’d, the na-tural surfaces of which must necessarily have been the most rugged of those of allFossils, from the irregular prominency of the several small Pebbles contain’d in them;yet this surface is frequently in the gravel pits of Hertfordssiire, especially in the smallerspecimens found quite smooth, and all the Pebbles worn down even, as if cut off byart: this is one great proof of the force of that vast body of water in departing;another is, that the Pebbles in the,deepest pits are frequently found naturally broken,and in halves, which could have happen’d only in that place, and by means of theirbeing dasti’d together at that time ; but this Pebble, in its Ætites form, is a yet greaterproof, for when broken we frequently find that the Callimus, for so the Nucleus nowloose is call’d, has, by means of this motion, beat off all the substance of the innercrust, and reduc’d it to a whitish powder, in which it lies half bury’d, and has a largecavity to rattle in. Many of the Ætitæ in this state have been naturally broken, andwassi’d about since, so that we frequently find only the single and empty halves of them.

5 . Calculussplendidus , nucleo fusco^ majori 3 crujlis albis fufcis.Shining , brown center d Pebble, with white and brown crujls.

This is a very elegant and beautiful stone, and tho’ it takes in only two colours, yethas those both so beautiful and pure, and is of so elegant a structure, as to be inferiorto few of the Pebble Genus; it is of an extremely fine, close, and compact texture, ofthe common wrinkled surface of Pebbles, of an orbicular or oval form, but generallysomething flatted, and frequently of four or five inches in diameter; it is compos’d ofa very large oblong central Nucleus, of a deep, but yet bright and agreeable brown,of an extremely fine texture, very bright and transparent, but subject, at times, to thewhitish specks so common in the larger Nuclei of Pebbles, tho’ in less quantitythan in most others, and often without them. The Nucleus is immediately surroundedby a moderately broad crust or zone, of a fine milky white,with somewhat of the pearlyblue in it, and that by a second of the same structure and colour with the Nucleus,except that it is more bright and pellucid; over these is the crust of the whole.The whole of this stone is very fine, bright, and transparent, but particularly thewhite circle, which is not at all inferior to many of the genuine Agates: This is thegeneral appearance of this species, but it is subject to the same varieties with therest, of having its veins less regularly difpos’d, or blending one with another, or bothwith the matter of the Nucleus, and making together but one plain and finebrown mass.

It breaks with a very beautifully even, bright, and glossy surface.

It is considerably transparent in thin pieces.

It is extreamly hard, and capable of a fine polish.

It very freely and readily gives fire with steel.

It makes not the least effervescence with Aqua Fortis.

It burns to a reddish white.

Examin’d by the Microscope, it appears of a very beautiful, even and close texture,and more bright and transparent than most of the other Pebbles ; its white veins arethe most so, and are indeed very elegant and fine.

It is found in many of our gravel pits, tho’ not so common as some of the former;I have observ’d it plentifully about Islington, and sometimes in the pits on Hampst eac *heath, but there more rarely.

6 . Calculus