SIR G. S. MACKENZIE on the Formation of Chalcedony. &5 



and, which is most remarkable, the size increasing gradually 

 from one end of the specimen to the other. We cannot imagine 

 that the heat varied so much in intensity within the space of a 

 few inches, as to render the Chalcedony in one part exceedingly 

 thin, and in another viscid. 



There is another supposition, with respect to this form, which 

 may be considered, while the former is before us in its most simple 

 shape. It has been suggested, that the pendulous form has been 

 produced in the same manner as a stalactite, by the successive 

 deposition of layers from a solution. This idea appears exceed- 

 ingly plausible, when we find, for the most part, this form pre- 

 senting concentric layers. But we know that such layers may 

 be produced by simple fusion in particular circumstances. For 

 instance, when a wax-candle is about to be made, the wick is 

 suspended in the centre of the mould, and the melted wax is 

 poured in at once. From such an operation we should not ex- 

 pect to find the candle formed of layers. Nevertheless, it is so ; 

 and by exhibiting such a candle, I can satisfy the Society of the 

 fact. This appearance of concentric layers or coats, is evidently 

 caused by the wick operating in some peculiar manner, so as to 

 give a particular direction to the process of crystallisation. In 

 almost all the cases in which the layers in Chalcedony are per- 

 ceptible, we find something analogous to the wick of the candle ; 

 either a hollow tube, or a thread of chlorite, zeolite, or some other 

 substance. In the only large specimen of pendulous Chalcedony 

 which I possess that has not such a thread within it, no layers 

 are perceptible, even with the assistance of cutting and polish- 

 ing. 



The strict parallelism of the layers of Chalcedony, in its dif- 

 ferent forms, cannot, I should think, be attributed to any other 

 cause than crystallisation, under the influence of some modifying 

 power ; and such facts as I have stated, seem to favour the idea 

 of a state of fusion, and to afford ground of triumph to its sup- 



