wit h a Coating of Chalcedony ." 343 



vered with this production ; the crust is unpolished and 

 rough to the tongue* imbibes the acids, and even sometimes 

 gives signs of effervescence. There are, to be sure, some of 

 them covered ujith a very hard chalcedony, but always 

 thicker, less transparent, forming an unequal crust, present- 

 ing only some brilliant particles in the fractures, where we 

 may perceive the traces of the friction, but never of the 

 gloss of enamel. 



One would think that the polish received by flint ought 

 to account for the gloss of chalcedony which covers it, and 

 produce the difference of the earthy chalcedony which covers 

 the two fractures; but the chalcedony observed upon several 

 rock crystals, upon mamellated agates, or that which we 

 meet with upon cubes of fluate of lime, or on other crystals, 

 the united surfaces of which may be considered as polished, 

 never presents any thing like a shining aspect. 



The stalagmite chalcedony of Geyser, in Iceland *, also 

 has no appearance of enamel even upon the surfaces which 

 have been in contact with flat bodies. The hydrophane 

 chalcedony, such as we observe forming passages in pech- 

 stones, petro-silex, &c, are also of a rough white, often 

 even in recent fractures ; besides, they are always found in 

 veins, and never in the form of a crust. It may be said of 

 them, as well as of opals, that their fracture, although of a 

 brighter lustre, is always unequal and undulating, and pre- 

 sents nothing to the eye approaching the lustre which po- 

 lishing gives them. 



Two pieces, however, presented to me a surface polished 

 enough to encourage the hope of finding the analogy of this 

 silex in nature. One of these pieces came from the depart- 

 ment of the Indre and the Loire; it appeared entirely co- 

 vered with white chalcedony; but having broken it, in order 

 to examine the interior, I saw nothing but a mass of the 

 same nature, which had no polish on its suiface, except 

 what it acquired bv rubbing ; a circumstance which excludes 

 all connection with the former. 



The other piece came from Siberia ; one of its sin! 



• See Bergman's xjuth Dissertation on Quartzy partita. 



Z i approached 



