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



pidly vanishing before the independence and candour of the pu- 

 pils of the Wernerian School itself. Many of them have gained 

 much credit to themselves, not merely for their careful examina- 

 tion of foreign countries, but from their having appealed to 

 the very scenes of their education, and acknowledged that they 

 now consider the views formerly impressed upon them to 

 have been erroneous. There is yet, however, a farther stretch 

 of candour to be expected from them ; and that is an acknow- 

 ledgment, that they had been anticipated in their views of 

 the origin of Trap-rocks, by the result of volcanic and trap coun- 

 tries having been examined by geologists of the Huttonian School. 

 The proofs of the igneous origin of such rocks, that were deposi- 

 ted in the cabinet of this Society more than eleven years ago, 

 will now be regarded as of more importance than what was then 

 conceded to them. 



The idea of HUTTON, that the materials of all rocks have been 

 so affected by heat, as to have become in some cases soft, and in 

 most cases fluid, is, as a general proposition, manifestly contra- 

 dicted by facts. His theory, however, as illustrated by PLAY- 

 FAIR, I have always considered as better fitted to explain a great 

 mass of facts than any that has yet been promulgated. But it 

 does not explain every thing ; nor can I see any reason for ap- 

 pealing either to heat or to water exclusively, for the consolida- 

 tion of the globe, while both agents may be called to our aid, 

 and while the power of gravitation is in continual action. I 

 have it in my power to satisfy the Society, that there is another 

 power sufficient to convert loose materials into solid masses. I 

 refer to the power usually denominated the Attraction of Cohe- 

 sion. I had occasion, a considerable number of years ago, to fill 

 a medicine-chest, that was to be sent into the country. Into 

 one of the drawers I pressed some magnesia ; but I cannot now 

 recollect whether it was or was not then, as it is now, in the state 



