THE "GREAT ICE AGE." 183 



that of finding sufficient water to float the ice, and should 

 have given it up had I accepted Mr. Geikie's estimate of 

 the thickness of the great ice-sheet of the great ice age. 



He says (page 186) that "The ice which covered the low 

 grounds of Scotland during the early cold stages of the 

 glacial epoch was certainly more than 2000 feet in thick- 

 ness, and it must have been even deeper than this between 

 the mainland and the Outer Hebrides. To cause such a 

 mass to float, the sea around Scotland would require to be- 

 come deeper than now by 1400 or 1500 feet at least." 



I am unable to understand by what means Mr. Geikie 

 measured this depth of the ice which covered these low 

 grounds, except by assuming that its surface was level with 

 that of the upper ice-marks of the hills beyond. The fol- 

 lowing passage on page 63 seems to indicate that he really 

 has measured it thus: 



" Now the scratches may be traced from the islands and 

 the coast-line up to an elevation of at least 3,500 feet; so 

 that ice must have covered the country to that height at 

 least. In the Highlands the tide of ice streamed out from 

 the central elevations down all the main straths and glens; 

 and by measuring the height attained by the smoothed and 

 rounded rocks we are enabled to estimate roughly the prob- 

 able thickness of the old ice-sheet. But it can only be a 

 rough estimate, for so long a time has elapsed since the ice 

 disappeared, the rain and frost together have so split up 

 and worn down the rocks of these highland mountains 

 that much of the smoothing and polishing has vanished. 

 But although the finer marks of the ice-chisel have thus 

 frequently been obliterated, yet the broader effects remain 

 conspicuous enough. From an extensive examination of 

 these we gather that the ice could not have been less, and 

 was probably more than 3,000 feet thick in its deepest 

 parts." 



Page 80 he says: " Bearing in mind the vast thickness 

 reached by the Scotch ice-sheet, it becomes very evident 

 that the ice would flow along the bottom of the sea with as 

 much ease as it poured across the land, and every island 

 would be surmounted and crushed, and scored and polished 

 just as readily as the hills of the mainland were." 



