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



it, as it is interesting in another particular besides that on ac- 

 count of which I now exhibit it. 



It is obvious that, if any change in the position of the pen- 

 dulous form has taken place, the fluidity of the mass could not 

 have been very great. I possess a great many specimens, the 

 forms of which cannot be accounted for in any other way than 

 by supposing that the cavities were in motion previously to the 

 Chalcedony becoming solid. In one of these the pendulous 

 masses have taken three directions. In another they appear as 

 if radiating from a centre ; and, in this one particularly, the form 

 of a dropping mass of viscid glass is very striking. In a third 

 we see the pendulous masses twisted in a variety of ways ; some 

 into the form of a cork-screw. In another cavity we have a few 

 twisted, but the greatest number are all bent in one direction. 

 The most beautiful specimen of this which I possess, shews the 

 pendulous form, after being fairly bent round, attached at both 

 ends to the side of the cavity. This has no appearance of two 

 pendulous masses having met and joined. The curve is quite 

 regular, and not in any degree altered by thickening, which 

 would have happened had two masses met ; nor is there the 

 slightest indication of the matter having made farther progress 

 downwards. Such a form as this could not possibly have been 

 produced from a state of extreme fluidity ; and hence there may 

 be drawn a strong argument for fluidity by heat ; and for pen- 

 dulous masses having, in some instances, been formed at once, 

 and not by a deposition of successive coats. But we are by no 

 means allowed to rest satisfied with such a conclusion as a general 

 one. I now present specimens which prove, most distinctly, the 

 matter of which they are formed to have been as thin, perhaps, 

 as water. Whoever has seen water trickling from above, amongst 

 grass or trailing plants, during frost, will have observed a forma- 

 tion of ice precisely of the form which Chalcedony has assumed 

 in this specimen ; which also proves that Chalcedony has been 



