198 HEAD OF FLINT. 



"But it docs not I n these collections, flints commonly appear encrusted 

 natural crusts ratner tnan covered ; and the crust is dull, adheres to the 

 of other flints, tongue, imbibes acids, and even exhibits some signs of ef- 

 fervescence. There are some, it is true, covered with a 

 very hard calcedony ; but it is always thicker, less transpa- 

 rent, forming an uneven crust, exhibiting only a few shin- 

 ing parts in the fractures where traces of friction are -percep- 

 tible, and never possessing the glassiness of enamel. 

 Cakedomes It might be supposed, that the polish of the cut flint 



smooth sur- occasioned the glassiness of the calcedony that covers it, and 

 faces have not produced the difference appearing in the earthy calcedony, 

 thTs^° hSh0f tnat hovers the two fractures: but the calcedony observed 

 on several rock crystals, on mamillary agates, and moulded 

 on cubes of fluate of lime or other crystals, the surfaces of 

 which may be considered as polished, never have this shining 

 aspect, 

 nor stalactitic. The stalactltic calcedony of Geysser, in Iceland, is equally 

 without the appearance of enamel, even on surfaces that 

 have been in contact with flat bodies, 

 nor hydrophar- The hidrophanous calcedonies, observed as forming tran- 

 nous calce- . sitions in pitchstones, chert, &c. are likewise of a dull 

 white, frequently even in the recent fractures. They are 

 nor opal. always found in veins too, never as crusts. The same may 



be said of the opals, the fracture of which, though of a more 

 vivid lustre, is always unequal, undulated, and exhibits no 

 appearance to the eye approaching the lustre given by po- 

 lishing. 

 Two calcedo- Two specimens however offered me a surface of sufficient 



nies with po- p p; sn t cf lXe no hone of finding in nature a silex analogous 

 lished surfaces r ' ° * n ° 



•were not to this antique. One came from the department of the 



crusts, Indre and Loire. It appeared entirely covered with white 



calcedony ; but on breaking it, to examine its interior, I 



perceived only a continued mass of the same nature, the 



, r h d stirnice of which had acquired its polish solely from friction, 



by friction. which excluded all comparison. 



The other specimen came from Siberia. One of its sur- 

 faces approached somewhat more in appearance a vitrified 

 enamel, and had a tolerable lustre : but it was quite as fo- 

 reign to flint and its transitions, since it was nothing but an 

 opake white calcedony, on a calcedony more transparent. 



It 



