323 



ing into cavities, previously existing in the bed, in which 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, respect- 

 ing operations performed in very distant ages, and where the ma- 

 terials 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 OF CARLSBAD. ...OF ICELAND. ...OF BATH.*.. 

 SILICIOUS TUFA OF THE G EYSER.... 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 in solution, in some aqueous men- 

 struum, it next becomes necessary to inquire, by what solvent this 

 solution has been accomplished. 



