438 P S A D U R I A.

It will not strike fire with steel.

It makes a violent effervescence with Aqua Fortis.

It burns to a pale reddish white. The masses of this Stone are frequently intersectedin various directions, with plates or veins of a purer and whiter Spar. -

Examin’d by the Microscope, it appears a congeries of obtusely angular Granulæ,debas’d from their crystalline purity in most places by an admixture of Spar ; these co-here no where, tho’ they frequently come into contact, but are lodg’d in a cementi-tious matter of an earthy Spar, which is alone the cause of the cohesion of the mass.

It is common in many parts of the kingdom, but the quantities brought to London,for use in building, are from Portland; it cuts easily, and is a very useful Stone inbuilding, bearing the weather well, and not being subject to sweat, as it is call’d inpavements.

The Granulæ, as well as the cement of this Stone, are disunited by Aqua Fortis,and when free from the cementitious matter, continue to ferment with it, whence it isevident that they have Spar in their constitution. The veins of Spar in this Stone arewhat have been narrow, perpendicular, and angular fissures in the stratum, fill’d up intime with that substance.

7. Psadurium friabile , granulis rotundis.

Friable Psadurium , with a round Gritt.

This is the substance so much talk’d of in the world, under the name of the Letter-ing Stone, tho’ that is an erroneous name, it not being found about Lettering in Nor-thamptonstiire, but about Ketton in Rutland.

It is a very remarkable and beautiful Stone ; it is of a lax texture, and appears, wherebroken, of a cavernous or porous structure ; but this is not really the cafe, the cavitiesseen there being not originally in the Stone, but made by falling out of the inner pastof its Gritt, which is usually loose, and falls out as soon as its containing shell is broken;it is moderately heavy, and has very little brightness ; it is in colour of a dusky brownishwhite, and is compos’d of a roundish Gritt, laid very closely together, and surroundedwith a cementitious matter of a terrene Spar; it is tolerably compact in the mass, butwhen reduc’d to small pieces may be crumbled between the singers ; it does not stactthe hands in touching ; water pour’d on it pretty readily finds the way into its pores.

It will not strike fire with steel.

It ferments violently with, and is in great part dissolv’d by Aqua Fortis.

It burns to a pale-whitilh colour.

Examin’d by the Microscope, its cementitious matter appears a considerably coatsterrene Spar, and its Granules are all true Stalagmitæ, or small crusted sparry bodies; theouter coats of which adhere so very slightly one to another, that they usually separate onbreaking : The particular structure of these bodies having been already defcrib’d amongthe Stalagmitæ, need not be repeated here.

It is not only found about Ketton in Rutland, but in many other parts of the kingdom,and is us’d in many places in building.

Sect. II. Impure Psaduria, consisting of sparry, crystalline, and

talcy particles.

1. Psadurium fragile , scintillans , albido-virescens .Greenish-white , brittle , glittering Psadurium .

This is a coarse and dusty Stone, its particles cohering so slightly as to be continual})'falling off in form of powder ; it is of a lax, open texture, moderately heavy, and J)1colour of greyish-white, with sortie faint admixture of greenishness ; it is full of a num'her of glittering spangles of a foliaceous Talc, and compos’d, beside these, of a mode-rately large angular Gritt, surrounded by a very lax and coarse earthy Spar ; It issoft, and easily broken in the mass, and when reduc’d to small pieces, readily mouldedto powder between the fingers, and a little stains the hands in rubbing ; water po liron it readily pervades its structure, and wets it thro’.