124 NATURAL HISTORY 
projection permit that any juice fhould cryftallize in fuch thin, 
equal, and continued plates as form thefe fheaths : it is an opinion 
attended with much fewer difficulties to think that they all hardened 
nearly at one time, and that the whole fhoot when uniform was 
produc’d at one effort, but where the ftru&ure varies, by fome fuc- 
cedaneous, direct, or undulating efforts in point of time following 
clofe upon one another ; that in the latter cafe the juices of which 
the mafs confided, gave way to the efforts in proportion as their 
different mixtures made them more or lefs fufceptible of the motion 
impreffed, the moft agile and pureft ftone flying off to the greateft 
diftance from the center, and the coarfeft and mold opake re- 
maining neareft to the center; and to this latter opinion I the 
more willingly adhere, becaufe (as I hinted before) thefe ftones are 
generally more clear at the point than at the root, and becaufe 
in many orbicular lumps of cryftal, particularly Fig. vn. I find all 
the middle of the lump (a, 6 , c,) opake, terrene, and cloudy, 
the ffieath next the middle faint and dufky white, the next fheath 
faint purple ; the third a brighter white than the firft, the next a 
wider feam of purple, but its tinge fainter ; the fifth a more di- 
ftinCt white, the next a tranfparent lift of cryftal in which the 
purple tinge was fcarce difcernable; the laft and outmoft of all, the 
pureft cryftal. In this fpecimen it is very obfervable that the white 
and the purple are alternately fixed in parallel angular fillets, the 
cryftal gradually forfakes the purple tinge as it advances to the ex- 
tremity, and the white increafes its purity in three degrees till it 
ends in a fourth of the cleared: cryftal, confirming (as I fhould 
think) what has been hinted before, that the purer the cryftal, the 
farther it proceeds from the center, whilft the more impure and flug- 
gifh parts of the mafs reft ftubborn and unmoved in the center of 
all. The texture of Fig. xxv. eftablifhes the fame truth ; at the 
firft effort, the pureft cryftal fled off and coated the circumference 
with hexagonal culpides; a fainter effort fucceeded, whereby the 
purer parts of the remaining cryftal were protruded lo from the 
center as to form a circle of fubdiaphanous rays in the opake 
white, 
sect. x. The variety of figures in which thefe bodies are found has been 
Theirfigure. already mentioned, and the caufe to which thefe figures are owing 
muft now be taken fome notice of. That thefe ftones have been 
in a fluid ftate, and thence paffed into their prefent folidity, muft 
evidently appear, by obferving, that the four firft figures, Plate XIII. 
page 1 1 9, are of the ftaladical kind ; that Fig. v, vi, vii, xxm, 
xxix, xxxm, plainly indicate their having fhot forth as from a 
center, protruding themfelves every way till they terminate in a 
point ; 
