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



porters. But, the advocates of solution may say, unluckily for 

 our opponents, we find cases in which threads of foreign matter 

 have not only not produced a concentric arrangement around 

 them, but have not interfered with the assumption of the paral- 

 lel form. As it is my purpose to do justice to both sides of the 

 question, I am bound to state that there is a most important dif- 

 ference between these two cases. In that of pendulous Chalce- 

 dony, there is nothing to influence the crystallisation but the 

 tube or thread, consequently all its tendencies are towards the 

 centre. In the case of parallel Chalcedony, the matter has been 

 acted on by the bottom and sides of the cavity in which it is 

 contained, and this, together with its own gravity, seems to have 

 exerted an influence superior to that of the foreign matter sus- 

 pended in the fluid mass. In many specimens, the parallel Chal- 

 cedony seems to have been formed by the fluid matter rising 

 from the bottom, as* it gradually filled the cavity, and to have 

 surrounded the pendulous matter previously formed. But, in 

 the example before the Society, it is evident that the shape of the 

 cavity has not exerted any influence in disturbing the production 

 of the parallel form, as it appears to have done in the case of 

 agates. Accident, however, has led me to the real structure of 

 the Chalcedony in such cases. The specimen which I now sub- 

 mit for examination, was cut from a mass which seemed, as in 

 the former specimen, to have surrounded the pendulous. The 

 lapidary's oil having insinuated itself between the layers (a proof 

 that the layers are not contemporaneous), when I heated the re- 

 maining portion of the specimen red-hot, the Chalcedony becom- 

 ing opaque and white, the charred oil afforded a display of the 

 actual structure ; and it is certainly not such as might have been 

 expected. On looking at a specimen in its natural state, we can- 

 not predicate any other connection between the pendulous and 

 parallel Chalcedony, than simple adhesion ; yet we see how a 

 mere accident can prove the uncertainty of our speculations. 



