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ascertain, whether corresponding impregnations of other substances 
with silex, are attributable to the influence of water, or of fire. 
Yonrs, &c. 
LETTER XXXI. 
SILICIOUS PEBBLES CONJECTURES AS TO THE TIME, AND MODE 
OF THEIR FORMATION CRYSTALLINE FLUID OF REAUMUR 
AGATINE NODULES THEORY OF THEIR FORMATION OPI- 
NIONS OP DR. HUTTON AND MR. PLAYFAIR AQUEOUS ORIGIN 
SUPPORTED. 
There 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 
attention, 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 circumstances, 
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 
