272 M. Brongniart's Observations on the Arborimtions In 



contrary, the opaque matter occupies a narrow central line, 

 which appears to be the canal itself by which the colouring 

 substance has penetrated ; and all round this central thread 

 there occurs a semitransparent layer, much thicker than a mem- 

 branous tube would be, and which appears, as in the green in- 

 filtrations, to be the result of the infiltration of the colouring 

 matter into the very substance of the stone. Notwithstanding 

 its greater resemblance to a plant, I therefore am still of opi- 

 nion that it is a mere infiltration. Thus the inquiries which I 

 have made, have not, as yet, enabled me to discover in calce- 

 donies well characterized plants, whether belonging to the 

 group of Confer V8B or to any other family. 



As to the mode of formation of these infiltrations in the inte- 

 rior of calcedony, it forms no part of my object to account for 

 it ; and I leave to mineralogists to discuss the manner in which 

 the small canals which are filled by the colouring matter are 

 formed, the solid or gelatinous state of the stone at this period, 

 and the nature of the matter introduced into it. My only object 

 was to shew, that, in most cases, if not in all, the vegetable king- 

 dom has nothing to do with these infiltrations ; in other words, 

 that they do not represent vegetables, and that their mode of 

 branching even proves that the canals which occupy their axis, 

 do not owe their origin to confervoid filaments, which these infil- 

 trations may afterwards have enveloped, and caused to disap- 

 pear. I know that the presence of confervoid vegetables in hot 

 springs, which generally contain silica in solution, might have 

 accounted for their presence in these stones ; but the vegetables 

 of hot springs are oscillatoriae, a kind of confervas, which, more 

 than any other, differs from the infiltrations of calcedony, in 

 having its filaments always simple, and most commonly straight, 

 or only slightly flexuous.* — Hist, des Vegetaux Fossiles, lere 

 livraison, p. 29- 



• We have observed vegetables in siliceous sinter from Iceland. Such spe- 

 cimens, when cut and polished, might, with the inexperienced, pass for calce- 

 donies ; and we believe such siliceous sinters are preserved in some cabinets, 

 as arborescent calcedonies or moeha-stones. — Edit. 



2 



