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at last they will quite close up. All the spaces between its mole- 

 cules being thus filled with silicious matter, which serves to ag- 

 glutinate the molecules in a compact mass, the stone, he concludes, 

 will possess a flinty hardness; and will exhibit nothing of a gra- 

 nular 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 sub- 

 stance, 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, be- 

 come 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 

 transmutations, which have given rise to this opinion, are easily 

 explained, by supposing a partial introduction of a silicious fluid, 

 in various quantities, 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 

 ia 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 



