IV. On the Formation of Chalcedony. By SIR G. S. MACKENZIE, 

 Baronet, F. R. S. LOND. & Edin. 



(Read January 21. 1822 J 



ALTHOUGH I possess a collection of specimens of Chalcedony, 

 made expressly with the view to enable me to form some plau- 

 sible theory of its formation ; and, although my cabinet contains 

 a variety of forms, perhaps unequalled in any other collection, 

 I have, for many years, refrained from describing them, because 

 the oftener I contemplated my specimens, the less able I found 

 myself to apply any single agent, so that its operations alone 

 could account for their forms. It is, indeed, very long since I 

 entertained the opinion, that no hypothesis founded on the se- 

 parate agency of heat or of water, could be satisfactory to spe- 

 culative minds ; and that geologists could never account for the 

 formation of individual minerals, or of the rocks containing them, 

 without combining the power of heat with that of water. But 

 even this combination, though it can be easily conceived capable 

 of forming many of the productions of the mineral kingdom, 

 leaves numerous phenomena unexplained. 



While we endeavour to account for the forms of Chalcedony, 

 we are not fettered by the appearance of animal remains, as in 

 the case of flint. It has indeed been supposed by some very 

 ingenious philosophers, that Chalcedony incloses some very deli- 

 cate vegetable forms ; but, while we know that various substan- 

 ces belonging to the mineral kingdoms, some of which are found 

 connected with Chalcedony, assume arborescent forms, we may 



