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



centre. This specimen proves that both forms have had the same 

 origin. 



I possess a number of specimens which shew considerable variety 

 in the parallel form, and some of which will be exhibited in con- 

 nection with other forms. In some specimens, the lines appear 

 as if the matter forming them had collected, or secreted into this 

 form while the whole was fluid. I remember having observed 

 some beds of trap in the Faroe Islands, which, when viewed at a 

 little distance, had the same appearance in respect to the nodules 

 of zeolite which they contained. The nodules appeared exactly as 

 if they had been arrested in their progress towards forming lines, 

 like flint in chalk, by the consolidation of the bed containing 

 them. 



I have exhibited a specimen of the massive Chalcedony, from 

 which I had detached the exterior shell, which, impressing its 

 form on the included mass, proves that the shell had been first 

 formed, and had become solid, before the hollow, which it sur- 

 rounded, had been filled up. 



No. 6. I now produce a specimen of a shell which has the 

 botryoidal form, and which, from some remains of quartz-crys- 

 tals, appears to have been lined with them. But on the Chalce- 

 dony we observe the impressions of crystals, which prove that 

 the former was not, as in the other case, solid before other matter 

 was introduced. 



No. 7. Sometimes the shell, and the Chalcedony filling the 

 interior, are different both in colour and texture. In this spe- 

 cimen the shell is opaline and bluish white, and the interior mas- 

 sive Chalcedony is blackish grey. 



The botryoidal form occurs in considerable variety of shape, 

 colour and size. Its origin is to be attributed partly to the sur- 

 face of the cavity containing it, but chiefly, I believe, to the 

 Chalcedony having, in its progress into the cavity, assumed the 

 spherical form, and to a number of such forms, while soft, 



