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will quite close up. All the spaces between its molecules being 
thus filled with silicious matter, which serves to agglutinate the mole- 
cules in a compact mass, the stone, he concludes, will possess a 
flinty hardness ; and will exhibit nothing of a granular appearance 
in its fracture. 
From such an impregnation of masses of various earthy matters 
proceed Egyptian pebble, various -coloured jaspers, agates, &c. and 
by such an introduction of silicious matter, in aqueous solution, or 
in a state of extreme division, into moderately soft, and rounded 
nodules, of different combinations of the earths, variously coloured 
by metallic oxides, and chiefly by those of iron, are formed those 
pebbles, whose external coat, differing from their internal substance, 
distinguish them from the small boulders of porphyry, jasper, agate, 
cornelian, marble, lime-stone, &c. which, detached from the rocks, 
and forcibly agitated by the waters of the sea, become rounded by 
attrition against each other. 
In the present advanced state of chemistry, it is unnecessary to 
dwell on the opinion which has been entertained by M. Patrin, M. 
de Carosi, and others, that chalk undergoes a conversion into flint, 
except for the sake of remarking, that all those apparent transmuta- 
tions, which have given rise to this opinion, are easily explained, by 
supposing a partial introduction of a silicious fluid, in various quan- 
tities, into porous calcareous earth. 
The numerous impressions of organic substances, such as shells, 
coralloids, spines of echini, &c. discoverable in the substance, and 
on the surface, of pebbles, prove these to have existed in a soft 
state ; and render it probable, that these bodies became involved 
in them whilst the pebbles were yet forming among the exuviae of 
marine animals at the bottom of the water. By the concentric 
line observable in many pebbles, the accumulation of successive 
coats, whilst in a soft state, is sufficiently proved ; and the various 
plum-pudding stones clearly point out the infiltration of a silicious 
