86 SIR G. S. MACKENZIE on the Formation of Chalcedony. 



be possible, by means of a few of the specimens I possess, the 

 condition in which Chalcedony must have been, previous to its 

 assuming the forms it now presents. This may appear to some 

 a superfluous labour, as it is acknowledged on all hands that 

 Chalcedony has been fluid. But the kind of fluidity has not 

 been agreed upon. Besides, I trust that what I have to remark 

 will not be altogether uninteresting. I have on all occasions 

 stated my conviction, founded on proofs which I have seen, and 

 examined over a large extent, and several of which are in the 

 cabinet of this Society, that the rocks in which Chalcedony is most 

 commonly found, are of igneous origin ; that is, that the whole 

 materials of which these rocks are composed have been placed 

 as we see them, by volcanic power acting under the influence of 

 particular circumstances, and especially of pressure. I shall now 

 endeavour, by an impartial examination of specimens, to ascer- 

 tain whether the condition of Chalcedony, before its consolida- 

 tion, was compatible with that conviction. 



The various forms of Chalcedony may be comprehended, per- 

 haps, in four general varieties, to which I may give names, with- 

 out any other object than to facilitate description. I will there- 

 fore distinguish them by the appellations, Massive, Parallel, Bo- 

 tryoidal, and Pendulous. In the term Massive, I include the 

 agate structure, and I use it merely, because, though all solid 

 masses (with a very few exceptions *) have the agate form, that 

 form is not always visible, till the eye is assisted by cutting and 

 polishing the stone. By Parallel, I understand that variety 

 which is combined with varieties of opal, &c. in straight layers, 

 and which proves that these substances have had a common origin. 



* I have but one specimen in which I cannot distinguish the agate forms even 

 with the help of polishing. 



