92 SIB G. S. MACKENZIE on the Formation of Chalcedony. 



The last specimen of this kind which I submit to the inspec- 

 tion of the Society, is that described by Mr ALLAN, in his ac- 

 count of the minerals of the Faroe Islands, and which is publish- 

 ed in the Transactions. The peculiarity which this specimen 

 presents is, that the coating of Chalcedony over the quartzy crys- 

 tals appears to become thicker, as well as the crystals themselves 

 larger, and the mass of quartz greater, as we examine from one 

 end of the specimen to the other. The coating of Chalcedony 

 has assumed the botryoidal form, and this gradually becomes 

 more enlarged and distinct as the coating becomes thicker. The 

 idea of the formation of this specimen, which may probably na- 

 turally occur on the first inspection, is, that the cavity, at the 

 time when the quartz was introduced into it, was in a position 

 having the larger end downwards ; and that in consequence of 

 this, the accumulation of quartz became greatest at that end. 

 But let us remark, in the first place, that the outer shell of 

 Chalcedony preserves a uniform thickness all round, except at 

 the very extremity of the smallest end. It becomes, therefore, 

 as necessary to account for there being no accumulation of Chal- 

 cedony at the lower end, as for there being an accumulation of 

 quartz. This last is not such an accumulation as we should ex- 

 pect either from the entrance of a solution, or of a substance- 

 fluid per se. In the first case, if the entrance of the fluid had 

 been gradual, we should have observed successive coats, or suc- 

 cessive crystallisations ; but here we have no succession of coats, 

 nor of crystallisations ; the crystals extend all the way from the 

 shell to their points without interruption. I am aware that crys- 

 tals are common, which show that they have been enlarged by 

 successive depositions of matter ; but it is not easy to conceive 

 how such an enlargement could proceed, while the fluid was 

 moving from one part of a cavity to another. As the accumula- 

 tion of Chalcedony over the quartz appears, if I may so express 

 myself, to have the same local ratio as to quantity with the 



