323 
into cavities, previously existing in the bed, in vrhich these bodies 
have been formed. 
Whilst endeavouring to trace the operations of nature in these 
processes, imagination, it must be acknowledged, has sometimes 
prompted the judgment ; but for this, I trust, I shall obtain an ex- 
cuse from you, as well as from Mr. Playfair. In inquiries, re- 
specting operations performed in very distant ages, and where the 
materials from which our information is to be derived, are really so 
few, and so unconnected, being snatched either from the wrecks of a 
deluge, or the ashes of a conflagration, conjecture must, if ever, be 
allowed to lend its aid, in the inquiries of science. 
Yours, &c. 
LETTER XXXII. 
SILICIOUS WATERS OP CARLSBAD OF ICELAND OF BATH 
SllilCIOUS TUFA OF THE GEYSER- ••••••VEGETABLE CALCULI..^*.. 
FLINT IN THE EPIDERMIS OF PLANTS. 
Seeing reason to conclude, from this circumstantial evidence, 
that the silicious matter deposited in secondary fossils, as well as 
in pebbles, &c. has been held iu solution, in some aqueous men- 
struum, it next becomes necessary to inquire, by what solvent this 
solution has been accomplished. 
