SIR G. S. MACKENZIE on the Formation of Chalcedony. 91 



things ; so I believe it to be impossible for us to penetrate into 

 the modifications which have obtained in producing the appear-' 

 ances we are now considering. We are satisfied of the existence 

 of a certain order of succession, and we may live to see the 

 cause which rendered the objects of our investigation fluid ; but 

 I apprehend we shall never understand the mode of action by 

 which the properties and characters of mineral bodies are kept 

 distinct, in situations and circumstances in which we might ex- 

 pect them to be blended together, so as to produce an anoma- 

 lous mass *. 



No. 11. In order, however, to illustrate this subject still far- 

 ther, I exhibit a specimen, in which the evidence of the quartz- 

 crystals having assumed their proper form, before the substance 

 deposited over them had become solid, is distinct. This sub- 

 stance appears to be opal or zeolite, as in the former specimen ; 

 and over it is a coating of very minute quartz-crystals ; thus pre- 

 senting an alternation similar, in order, to the other case, but 

 having the crystallisation of the quartz more perfect. 



No. 12. Another specimen of this alternation, to which I re- 

 quest the attention of the Society, is a very remarkable one. The 

 quartz-crystals are distinctly formed, and a slight coating of Chal- 

 cedony appears over them. But above this is deposited a white 

 dust, some of which adheres firmly to the crystals, while the rest 

 is loose. This suggests the idea, that the cavity had been filled 

 with vapour, which had condensed into the form of this dust. 



No. 13. In the next specimen the shell is thick, and is lined 

 with a soft white substance. On looking at these specimens, the 

 advocates of the igneous theory may probably perceive something 

 not unlike the process of Sublimation. 



* Among numerous examples, well known to mineralogists, I may mention one 

 specimen in my possession, not much larger than a man's fist, which is formed pf dis- 

 tinct crystals of brown quartz, white topaz, and beryl. 



M 2 



