I0 8 NATURAL HISTORY 
principle which forms thofe of a cryftal bafe, that principle being adven- 
titious, not natural to either : however that be, it mull; not be ima- 
ged, that the reverfe will hold good*; for we have many factitious 
pebbles (of Porphyry for inftance, and others of cryftalline bafe) 
which will not ferment. To thefe criterions fome authors add, 
that they break irregularly * ; whereas faditious pebbles break 
to faces. How this natural figure therefore comes to corref- 
pond fo perfectly with that which is factitious , muft be our 
next enquiry. A modern author oblerves, that one part of the 
pebbles is more compreffed than the other, which he therefore 
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 
ftate of foftnefs and liquidity ; that, floating in a fluid medium be- 
fore they were hardened, they were rolled to and fro, if that me- 
dium was in violent motion, fo as to become round ; if that medium 
was in little motion, then they only became roundifti ; if at perfed 
reft, then ftill more flat. Thefe are ingenious conjectures, and the 
author ’ deferves commendation ; but there muft have been feveral 
other concurring caufes : To what is here fuggefted then, let us 
add the equal preffure of the fluid medium on all fides upon the 
ftone-maffes during their liquefcent ftate ; for we know that water 
will form melted lead, properly mixed, into a globular figure : 
Again ; the mutual attraction of fimilar parts will form fluid bodies 
into a roundnefs, as we may fee by drops of dew and quickfilver. 
All thefe caufes muft, I fliould think, be admitted ; nor are we to 
forget, that the innate force of the materials, the peculiar fait, ful- 
phur, mineral, or metallick earth, of which the ftone is compofed, 
muft have been fupple, and complying with the other forces, and 
not have any tendency to angular or rectilinear figures, or pebbles 
could not have been rounded at the time of their concretion. 
Thus much for the natural and factitious fhape of pebbles, the 
latter being occafioned by the agitation of water, and the natural 
formed in water, partly by its fluctuations and equilateral pref- 
fure, and partly by the mutual attraction and conlent of the mole- 
culcz , of which fuch ftones are compofed. As to the other proper- 
ties of pebbles, whereby they differ from one another, I ftiall only 
hint, that if the component parts were fmooth, fine, and fmall, 
the body formed of thefe will be of a clofe texture and glofly fur- 
face ; if we find the inward ftruCture uniform, it will intimate the 
unmixed nature of the materials ; if in layers of different earth 
and cement, we are to impute them to fucceflive applications of 
different fubftances, indurated at different but no very diftant times 
* That all pebbles, which will not ferment, are y Hill’s Hiftorv of Foffils of the Telaugium, 
natural ones. page 555, 
* Dr. Woodward’s Catalogue, vol. I. page 29. 
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