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



sumes the crystalline form. The Chalcedony and Quartz are dis- 

 tinctly separated by a thin film of a pale-bluish colour. Over 

 the quartz has been formed a layer of a white substance, proba- 

 bly opal or zeolite. Lastly, over this has formed a coating of 

 quartz, which shoots its crystals into the cavity. 



These successive, and varied formations may be supposed by 

 some to have been produced by different fluids, introduced into 

 the cavity at different times ; and by others as a modification of 

 crystallisation from one fluid. We have already seen the prac- 

 ticability of separating an exterior shell from the matter that has 

 filled up the cavity. In the present case, the shell has assumed, 

 as is usual, the botryoidal form ; and there appears nothing to 

 lead to the supposition that it had not become solid before the 

 next layer, which is of quartz, had been formed. The quartz of 

 the second layer appears, evidently, to have formed its crystalline 

 points ; and these points have caused the third layer of opal or 

 zeolite to assume the botryoidal form. Now, it is not, perhaps, 

 possible that, during the crystallisation of different substances 

 from one fluid, any layer with the points of the crystals formed, 

 except the last one (that in the interior of the cavity), could ex- 

 ist. On the other hand, it may be argued with equal plausibi- 

 lity, that, when once a hard shell is formed, covering every part 

 of a cavity, it is impossible to conceive how any fluid could enter 

 to form a second layer ; and the difficulty of conceiving this be- 

 comes always greater, the more numerous and varied the layers 

 become. Other arguments, on both sides, might be stated, and 

 illustrated by specimens ; but as these do not appear to prepon- 

 derate more to one side than to another, I will not at present 

 pursue the subject. I have said enough to prove, that it is no 

 easy matter for us to comprehend the operations of Nature. As 

 it is impossible for us to define the difference between that which 

 is material and that which is immaterial, or to know any thing 

 at all of the nature of matter, without knowing the essence of 



