342 Rev. 0. Fisher on Theories to 



about five miles, after which it becomes insensible. We find 

 at successive thousands of feet : — 



Depths . . 

 Cooling. . 



. 0. 

 . 20° 



1. 



18°-2 



2. 

 16° -4 



3. 

 15°-3 



4. 

 13°\L 



5. 

 11° -4 



6. 

 10°-2 



7. 

 9°-8 



8. 

 8°-8 



Depths . . 

 Cooling.. 



. 9. 

 . 6°3 



10. 

 5°-3 



11. 

 40.4 



12. 

 3°6 



13. 

 3°0 



14. 

 2°-4 



15. 

 l°-9 



16. 

 l°-5 



17. 

 l°-2 



Depths... 

 Cooling.. 



18. 

 . 0°9 



19. 

 0°-7 



20. 

 0°6 



21. 

 0°'4 



22. 

 0°3 



23. 

 0°-2 



24. 

 0°2 



25. 

 0°-2 



26. 

 0°1 



The resulting depression of the surface, even with the pro- 

 longed duration of an ice-sheet for 100,000 years, would 

 therefore amount to no more than 3*1 feet. 



The conclusion to be drawn from our investigation is in- 

 evitable — that no appreciable part of a glacial submergence, 

 known to have reached several hundred feet, can be accounted 

 for by the cooling of the rocks by the ice-sheet. The reasons 

 why the contraction and consequent depression of the surface 

 would be so small are to be found, firstly, in the extreme 

 slowness with which, if we may be permitted so to express it, 

 the cold would be conducted into the ground ; and, secondly, 

 the very small contraction which rocks undergo owing to 

 change of temperature. We may evidently dismiss this 

 theory of glacial submergence. 



Since, however, their coincidence renders it highly prob- 

 able that the connexion between glaciation and submergence 

 has not been fortuitous, but that they are related as cause and 

 effect, we will shortly refer to two other theories which have 

 been proposed. These are, (1) a depression of the surface of 

 the crust by the weight of the ice, so that the water of the 

 ocean should overflow the depressed area ; and (2) an eleva- 

 tion of the sea-level in the neighbourhood of an ice-sheet by 

 the attraction exercised upon the water by the mass of the 

 ice. We cannot form any opinion upon the efficacy of these 

 supposed causes of submergence without making a preliminary 

 supposition regarding the condition of the interior of the 

 earth ; for, unless that is sufficiently mobile to yield freely to 

 the weight of the ice-sheet, the first of the above causes could 

 not act ; while, on the other hand, unless the ice-sheet is 

 rigidly supported, the second cause would have only a slight 

 effect upon the level of the ocean. 



First, then, if the substratum, upon which the crust of the 

 earth rests, yields freely to any increase of pressure, so that 

 the crust is continually supported in hydrostatic equilibrium, 

 it is clear that any additional load laid upon a portion of the 

 surface would displace an equal mass of the substratum. 



