SIR G. S. MACKENZIE on the Formation of Chalcedony. 85 



of carbonate. Some years afterwards I opened this drawer, and, 

 to my great surprise, I found the magnesia as hard as chalk. I 

 sent pieces of it from the country to my friends ; but, from my 

 having neglected at the time to commit the fact to writing, it 

 has been forgotten, and the specimens have been lost. 



I have, however, the satisfaction to find that a quantity yet 

 remains in the drawer, which, though small, will be sufficient to 

 demonstrate the fact, that the mere juxta-position of the par- 

 ticles of magnesia is sufficient for consolidating them. Gravita- 

 tion, the attraction of cohesion, and the percolation of cold wa- 

 ter, containing such substances as it is capable of dissolving, may 

 account for the consolidation of many stratified rocks, though 

 certainly not for that of all. Water greatly heated may affect 

 much more as a solvent, and also by communicating heat. No- 

 thing, however, in my opinion, can account for many other phe- 

 nomena, but that the materials forming the rocks had been in a 

 state of actual fusion. While we suppose actual fusion to have 

 taken place, we may, without any risk of being considered fanci- 

 ful, believe that water may have been present in greater or less 

 quantity ; and that, when strongly heated, it has the property 

 of forming combinations, which it refuses to form when cold. Let 

 geologists, therefore, as they are at liberty to do, call to their aid all 

 the powers which nature presents, and cease to divide themselves 

 into parties. They have but one object in view, the discovery 

 of Truth. At present, geological theorists may be compared to 

 an army advancing against a fortress, with nothing but balls on 

 the right, nothing but guns in the centre, and nothing but pow- 

 der on the left. The three bodies make separate and indepen- 

 dent attacks ; but the arm of each is useless singly. They must 

 unite all the arms together before any impression can be made. 



It is not my intention, in this memoir, to enter much into the 

 consideration of causes. I propose to iUustrate, as far as it may 



