SIR G. S. MACKENZIE on the Formation of Chalcedony. 108 



stance in the form of dust or flocculi, which most probably had 

 condensed from a state of vapour. 



When Chalcedony is heated, splinters are thrown off with ex- 

 plosion ; a fact that indicates the conversion of some ingredient 

 of its composition into vapour. The escape of a component part 

 is indicated also by the chalcedony becoming opaque. It is most 

 likely that the matter which escapes is water ; and, indeed, I 

 have no doubt that water has had a chief concern in the forma- 

 tion of Chalcedony, and also of opal, and of all the varieties of 

 zeolite. That heat has operated along with it cannot be doubt- 

 ed, since we possess unequivocal proofs of its having affected the 

 masses of rock in which Chalcedony is for the most part found 

 I possess one specimen of the rock through which the Chalcedo- 

 ny appears to have become solid, while it was collecting and pas- 

 sing through the rock, having taken the branching form of blood- 

 vessels. This proves, that both the matter of the rock, and the 

 Chalcedony, had been soft and fluid at the same time ; and this 

 precludes the idea of the formation of cavities, in the first place, 

 and the subsequent gradual infiltration of a solution, if a thou- 

 sand other facts did not contradict it. The exclusion of heat as 

 an agent in the formation of the crust of the Earth, while so 

 many phenomena warranted its assumption, has greatly contri- 

 buted to multiply absurdities in the aqueous theories of the 

 Earth, while the exclusion of water has hindered HUTTON and 

 his illustrator from rendering the igneous theory the most per- 

 fect that has appeared. It is probable that water heated under 

 a powerful compressing force, will be found sufficient to explain 

 many anomalies both of the aqueous and igneous theories. But, 

 as I stated at the beginning of this memoir, geologists ought not 

 now to limit themselves to any one of the numerous agencies of 

 Nature. When geology was beginning to assume the form of a 

 science, it was of advantage, in the excitement of research, that 

 there should be theoretical parties, however absurd the word 



