SIR G. S. MACKENZIE on the Formation of Chakedony. 89 



uniting. The pendulous form seems to be only an enlargement, 

 or rather a prolongation, of the botryoidal ; and, to illustrate 

 this, I exhibit a beautiful specimen, having a pearly lustre. This 

 lustre is given by zeolite, or the substance called Cachalong, 

 forming the nucleus, or the thread, on which the Chalcedony has 

 been formed. The connection of the botryoidal with the pen- 

 dulous form is apparent. 



No. 8. I now exhibit a specimen, in which the massive and 

 pendulous forms are united ; and there are other specimens now 

 on the table, which show all the forms in a small space. Hence, 

 we may refer all the forms to one and the same previous condi- 

 tion of the substance of which they consist. That condition has 

 evidently been fluid ; but the kind and degree of fluidity is yet 

 to be investigated. Before proceeding farther, I may exhibit 

 the diversity of appearances which we meet with in the cavities, 

 which are formed by an exterior shell of Chalcedony. 



It often happens that we find nothing within the shell, 

 which varies much in thickness ; being sometimes scarcely thicker 

 than paper, and in other cases several inches in thickness, of 

 which several specimens are on the table. 



No. 9. In the next specimen there is imperfectly crystal- 

 Used quartz within the cavity ; and we observe the impressions 

 of other crystals deeply inserted, but not penetrating to the 

 Chalcedony. The crystals that have made their impressions 

 were probably of calcareous spar. 



No. 10. Another specimen, although connected with the 

 pendulous form (which is always contained in cavities) serves to 

 shew a remarkable series of -depositions. We observe, first, that 

 a dark-coloured mass, probably a portion of the rock in which 

 the Chalcedony was contained, has assumed the pendulous form. 

 Around this, the Chalcedony forming the exterior shell of the 

 cavity, has formed a pendulous mass. The next coating is 

 formed of quartz, which, as it recedes from the Chalcedony, as- 



VOL,. x. P. i. M 



