Dendritic Calcedony^ or Mocha-Stone. 271 



which the infiltration has been formed. There is also always 

 seen, in the middle of the filaments, a more opaque line pro- 

 duced by a matter of a deeper green. This line appears to re- 

 present the small canal itself which traverses the calcedony. It 

 is irregularly bulged at intervals, and the greenish nebulosity, 

 formed by the infiltration of the colouring substance in the 

 stone itself, has followed all the irregularities of this canal. 



The more or less linear or strongly mammillated form of these 

 infiltrations, and their greater or less opacity, appear to depend 

 upon the extent of these canals, and the quantity of colouring 

 matter which was contained by them. The aspect of these in- 

 filtrations also, the frequent anastomoses which they form, and 

 their irregularity, preclude all idea of vegetable origin. Some 

 gelatinous and tremelloid plants, such as the LincMcB, Mesa- 

 gkna^ &c. have somewhat of this appearance, but they never af- 

 fect this filamentary and anastomosing disposition, nor does ex- 

 amination with the microscope enable us to detect any analogy ; 

 for, with a magnifying power such as that which we employed, 

 these plants presented characters of structure which immediate- 

 ly distinguished them. 



These two forms of infiltration are those which are of most 

 frequent occurrence in moss agates. I have observed another 

 in a part of a slice filled with brown infiltrations, which left 

 some doubts on my mind as to its origin. It presents a regu- 

 larity in the filaments of which it is composed, and in their mode 

 of division, which pretty distinctly suggests the idea of several 

 confer vae, and, in particular, of certain species of the genus 

 Bangia^ such as B. atrovirens *. It is, however, possible, that 

 it may be nothing else than a more regular infiltration, produ- 

 ced by very minute and regularly ramified canals. What would 

 induce me to think so, is the mode of distribution of the opaque 

 matter toward the centre of the filaments. In the confervoid 

 plants, to which these filaments might be compared, the gra- 

 nular and coloured opaque matter fills the whole cavity of a thin 

 and membranous tube. The transparent part, which is desti- 

 tute of this granular matter, therefore forms on the edges but a 

 narrow border, produced by the wall of the tube. Here, on the 



• Lyngbye, Tent Hydroph. Danicse, tab. xxv. fig. B. 



