io8 N A T U R A L , H I S T O R Y
principle whichforms thofeof acryftàl bafe,that principle being^dyenr
titiôus, not natural to either: hovifeVer that be* it; muft not?be imagined,
that the reverie will hold good*; for we have many factious
pebbles (of Porphyry for inftance, and others!of cryftalline bafe)
which: will'not ferment;, .To thefe feriterioiis 'feme' authors add,
that they break irregularly ;x ; - whereas factitious pebbles break
to faces. How this natural figure-th’erefpEeif conaes, to côrref-
pond fo IperfeCtiy with that wh\ch *is .faCtitfeus,., muft .be- opr
next enquiry. A modern author .flbferves, ■ that one part of. the
pebbles is more compreffed than the other,-.which he thereforè
rightly terms their bafe, and argues very juftly, that the lumps of
which, they are formed, muft have been at one time or other in a
ffate of foftnefs and liquidity ; that, floating in a fluid medwmuhgs-
fore they were- hardened, they were rolled; td.and. fro, if th?it.. ine^
dium was in violent, motion*, % as.-to become, round ; if :that medium
waa in little motion, .then they only.became roundifh ; ife&fcperfé<|
reft, then ftill more flat. ; : Thefe are ingenious conje<|:uresj '.and the
author’ deferves commendation; butthere.mufthavéîheen' fevér-td
other concurring caufes: T o what is here fuggefted th e n ^ ^ u s
add the equal preflliire o f the fluid medium-on., all fides; ..Upoh the
ftone-maffes during their-liquelcent ftate ; for we know that water
will form melted lead, i properly mixed, into a globulaj ^figurp,:
Again ; the mutual attraction of fimflar parts v^ill form-fluid .-bodies
into a roundnefs, as we may fee. by drops o f dew and* qüickfilvei'-.
A ll thefe caufes muft, I Ihould think, be admitted ; nonaref^np
forget, that the innate force of
phur, mineral, or metallick earth, ..of which the |tone is^cpmpqfed,
muft have been fiipple,. and .complying with the ^dflier^fôrce.s', and
not have any tendency to, angulai or rectilinear figures, or
could not have been rounded at the time of thfir eoneretfeni!..,.
Thus much for the natural and. factitious fliape of pebbles, the
latter being occafioned by the agttation of. water, and, |hf natural
formed in water, partly by. its fluctuations and equilateral pref-
fiire, and partly by the mutual attraction and cpnfent.qf the molecule,
of which luch ftones are compofed. As to the other properties
o f pebbles, whereby, they differ from one another, I fhall only
hint, that if the component parts were Imooth, fine, and fmall,
the body formed of thefe will be o f a clofe texture and glofly fur-
face ; if we find the inward ftruCture uniform, it will intimate the
unmixed nature o f the materials ; if in layers .■ of different earth
and cement, we . are to impute them to fucceffive applications of
different fubftances, indurated at different but no very diftant times;
* That all pebbles, which will not ferment, are r Hill’s Hiftory of Foffils of the Telaugium,
natural ones. ' ‘ . . page 55$,
. * Dr. Woodward’s Catalogue, vol. I. pigé 29.
if
° : FV C O R N -W A L L. 10g
interfeded, by feams.oflftbne different from the body,
rhe^ëafon pofWyAfiajrbe this ; that f a i^ ë body was contracting
itfelf in o^er^odndui^on'^cdntraCfiqrt cit approxirimtion:of. parts
being ■ a.rrieeeflkry1 concomitant 'óf'indttf-ktionj, or afljslffinduration
cleaved by force of fire or accidejhfl^'fiflbVe enfuing, that fiflure
was filled with ;thej,‘adjoining matter,^ repelled by thé fubftaÜde o f
the body, and formed óf its’ own Uniform parts by mutual attraction
betwixt themfelvescand exfclufion o f othets. I f therff’are uneven-
nelfes iffthe furface of pebbles (a'S^as the caféVïth many, which
made Dr. Woodward'recur to thé' agitation of the waters Of the
deluge) feme parts being'more prorifttient than others, or if there
are -loofe nucleus^iriclöfed within the central' ckvity, .< as is the bafeff'
e f’ the ’.SLtites,?, it need- only fee fuggeftèd', that thefe phafeomina
n^jeafoflably be attributed to thé differétit èöntfaCtile powers o f
the materials of which thefe unevenneffes and central nucleus’s con-
fift-; it being certain, that if the bódy of ’a, ftóne-contracts itfelf'
in order to induration, into a clofer Tubftancè than the feaiti; feptkni,
or granulated charge, ^zt-icharge or feam will- be more proitiinent
than thé'body, • and vice verf a ; and if the fubftance of the nucleus;'
during the time of induration, contracts; itfelf after and more than
the fubftanceof the fhell which invefts it, that nucleus will have a
vacancy round "it/ and become loofc in its cell, the* nucleus being a
iqoncretion pofteriour fo that of the fhell, ai^ 'brrakirig’ loofe froiff
the inner** coat ^ of thé fhell hy thecontraCtile powêrs’ of its own
cönftituent parts. Laftly, of pebbles : Some are evidéhtly formed
fince the deluge, for we find fhells, coralloid bodies, and echinites
in thefn; ' whether thofe which carry no filch* evidences wete formed
before, at, or long fince thé'deluge,- it is impoffible to ‘determine.
Fjóm pèbbles, let us pafs 'to nodules of »curious ftones found in sect.iv.
Cornwall. Having found feme pebbles of porphyry pn the fea-Modules-
fhore, upon farther fearch I difeovered a ftone of the fame kind .
in the parifh of Philac, among the land-hills: it is of a ruddy-
purple ground (not fo red as the Egyptian) charged with granules
rectangular and oval, from the eighth of an inch diameter and under,
nearly of the lame colour with the ground, but paler with glofly
forfaces, interfperfed thinly with opake white granules of quartz of
like fize and fhape to the foregoing, mixed with feme black fpecks of
cockle of the fame fize. It weighs to water as 2 - to 1 *•
N». ii. Part of a large nodule o f like porphyry found in my
garden, had its granules larger, but no white interfperfed. One
v? One of which, of a hard brown flint-like " and brought me September 17, 1757.
ftone, about an inch in diameter, was found m 'l'p * This feems to be thepophyrites leucofti&tos
*757) near Penzance Kaye, among the fen-pebbles, of the ancients.
F f pebble