2 Sir G. S. Mackenzie on the most recent 



slow but enduring forces may have produced such changes in 

 the relations of the earth to the sun, as would have caused 

 great cold on the surface. But such appeals to account for 

 ice — or, to account for a debacle, such an idea as that of a 

 comet coming into collision with the earth, suddenly changing 

 its axis, and giving it a new equator, are scarcely to be admit- 

 ted, while there remains a chance of finding geological causes 

 sufficient to solve our difficulties. Astronomical changes are 

 slow, and I apprehend we need to find means for a sudden 

 production of cold and ice ; for, when we contemplate the pre- 

 sence, in regions beyond the tropics, of the remains of ani- 

 mals which required a temperature approaching to that with- 

 in them, we see no indication of any but a sudden change. 

 Hence came the notion of a catastrophe caused by a comet. 



The cause which might have effected a reduction of tem- 

 perature in the crust of the earth, to a degree that would ac- 

 count for the surface having been formerly covered with ice, 

 or at least for the glaciers of Switzerland having, at a former 

 period, stood much higher and extended much farther than at 

 present, is yet in obscurity. It is with some diffidence that 

 I venture to submit to the Society what has occurred to me 

 on this interesting subject ; but being persuaded of the im- 

 portance of exciting the minds of others into a train of think- 

 ing that may perhaps lead to the development of ideas more 

 plausible than my own, I do venture, in the hope that my 

 conjectures may elicit from other minds something more sa- 

 tisfactory. I have been anxious to discover whether any such 

 effect as a considerable refrigeration of the surface, and the 

 production of ice, might not have followed some of those dis- 

 turbances of the rocky crust, w^hich are so plainly indicated 

 by the present relative position of the broken masses. 



The evidence that the glaciers of Switzerland have stood 

 at a higher elevation, and that they had extended much far- 

 ther than they do at present, seems to be complete. That 

 their reduction in elevation and extent had been caused by an 

 increasing temperature of the earth and the influence of the 

 sun combined, can scarcely be doubted. I had therefore to 

 look first for some indications rendering it probable that the 

 eartb^s cni»t, with reference to temperature, had been, pre- 



