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



cent, of alumina in one specimen, and 2 per cent, in another, to- 

 gether with 1 1 per cent, of lime. May not such variations in 

 composition cause variations in the effects of heat applied to 

 Chalcedony ? 



It is evident that the degree of heat necessary for the fusion 

 of Chalcedony, is not so high as we might be apt to imagine : 

 for in the specimens I now present, we find that Chalcedony has 

 covered calcareous crystals without altering their form. We 

 know, from the experiments of Sir JAMES HALL, the degree of 

 heat necessary for the fusion of carbonate of lime ; and from the 

 specimens it is clear, that the temperature required for the fu- 

 sion of Chalcedony, had not the power to melt carbonate of lime. 

 If, on the other hand, we are to call in the aid of a solvent, these 

 specimens prove that the solvent of Chalcedony could not act on 

 carbonate of lime. But it is useless to enter inte a maze of sup- 

 position. I shall only observe, that I am rather puzzled, when 

 I see in one specimen Chalcedony covering calcareous spar, and 

 also calcareous spar covering Chalcedony. This is an anomaly 

 which, in the present state of our knowledge, cannot well be re- 

 conciled to contemporaneous formation, whether we suppose heat 

 or a solvent, or both together, to have been in action. Never- 

 theless, I am disposed to consider the formation to be contem- 

 poraneous ; and possibly when we come to be better acquainted 

 with the power of water greatly heated, the mystery may not ap- 

 pear so great. 



There is yet another mode of formation to which some forms 

 of Chalcedony appear to have considerable analogy. I refer to 

 the formation of pendulous masses from vapour, of which the 

 best illustration is to be seen in the singular substance sometimes 

 found 'in the retorts of gas manufactories. That substance pre- 

 sents to us the botryoidal as well as the pendulous form; a 

 combination very common in Chalcedony. I have already pro- 

 duced a specimen in which the cavity is sprinkled with a sub- 



