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Ohservatimis on the Arborizations in Dendritic Calcedony, or 

 Mocha Stone. By Ad. Brongniakt * 



F OR a long time, those irregular filaments observed in the 

 varieties of agate commonly called Moss Agates, were believed to 

 be confervoe. Daubenton was the first who made known his 

 opinion on this subject f ; and while he admitted the arboriza- 

 tions of agates to be mere infiltrations, he thought he distin- 

 guished in certain moss agates real vegetables, and in particular 

 confervae and mosses. The figures which he has published of 

 these objects are too imperfect to enable us to form any idea 

 respecting the forms which he intended to represent, nor have 

 I been able to find any thing resembling them among the speci- 

 mens preserved either in the old collection of the Jardin des 



• The following observation on mocha, stone occur in mj System of Mine- 

 ralogy. 



" The dendritic calcedonies, or mocha-stones, are much prized as ornamen- 

 tal stones. The arborizations, as already mentioned, are black, brown, or 

 green. The black as the most common, and most distinct : the red, on the 

 contrary, are rarer, and are less distinct, and are named corallines, from the re- 

 semblance of the dendritic delineations to coral ; and the green are rare, and 

 much esteemed. These arborizations appear in some cases to be owing to 

 iron, in others to manganese, iron, and mineral oil. Deutens, Von Moll, 

 Daubenton, and lately Lenz, Blumenbach, and Dr MacCulloch, maintain 

 that many of them are of a true vegetable nature. Deutens says, that if the 

 plants contained in calcedony are extracted, and the fragments thrown on 

 burning charcoal, a bituminous smell is exhaled ; and Von M oil maintains, that 

 calcedony sometimes contains brown and green moss. 



" Lenz affirms, that the calcedony found in the amygdaloid of Deuxponts 

 contains musci of different kinds, such as Lichen rangiferinus, Confervae, Byssi, 

 and Brya. And Blumenbach says, in a letter to Baron Von MoU, that though 

 he had hitherto disbelieved the occurrence of vegetable bodies in the dendritic 

 variety of calcedony named mocha-stone, he must now admit that it does 

 sometimes contain plants, apparently of the nature of conferva. He observed 

 these in specimens from Iceland and Catharinenburg. The same celebrated 

 naturalist maintains, that he found, ^in the interior of an agate, the fructifica- 

 tion of an unknown plant, somewhat resembling the Sparganium erectum. Dr 

 MacCulloch, after examining several hundred specimens of mocha-stone, is of 

 opinion that they contain cryptogamous plants. This opinion, however, still 

 remains very improbable."— «/aOT^5o«.'s Mineralogy, vol. ii. 



t Memoires de P Academic des Sciences 1782, p. 667. 



