311 



ascertain, whether corresponding impregnations of other substances 

 with silex, are attributable to the influence of water, or of fire. 



Yours, &c. 



LETTER XXXI. 



SILICIOUS PEBBLES. ...CONJECTURES AS TO THE TIME, AND MODE 

 OF THEIR FORMATION. ...CRYSTALLINE FLUID OF REAUMUR.... 

 AGATINE NODULES. ...THEORY OF THEIR FORM ATION.... OPI- 

 NIONS OF DR. HUTTON AND MR. PLAYFAIR.-.. AQUEOUS ORIGIN 

 SUPPORTED. 



1 HERE is hardly any one of the combinations into which silex 

 enters, but displays some wonderful phenomenon. The sand and 

 pebbles which we tread upon, and which so seldom attract the 

 attentipn, are fraught with mystery. Henckel, with mingled per- 

 plexity, and wonder, exclaimed, " O Caillou ! Caillou ! qui est ce 

 qui t'a engendu ? Est ce done qu'il ne seroit possible de se faire 

 quelque idee de la maniere que cette formation se fait ?." , 



Almost without a hope of adding to what is already known, 

 respecting these substances, I shall lay before you a few thoughts 

 on the formation of silicious pebbles ; hoping, that I shall thereby 

 at least, make some advancement in a knowledge of the circum- 

 stances, on which the silicious impregnations of wood depend. 



Various opinions have been entertained as to the aera of the for- 

 mation of pebbles: some believing their existence to be coeval 



