548 James Geihie — On Changes of Climate. 



the Till, a broad sea would separate the few tops remaining above 

 water from the Grampian mov;ntains, whence no small proportion 

 of the stones in the Till referred to have been derived. How, then, 

 could this Boulder-clay be deposited from the seaward margin of the 

 ice-sheet ? In this instance the exploded iceberg theory has more to 

 commend it as an explanation of the facts than the hypothesis under 

 review. This latter hypothesis is no less irreconcilable with the 

 appearances presented by the Boulder-clay of the southern uplands. 

 Let those regions be submerged so as to bring all the Till-covered 

 portions below the level of the sea, and we shall find only a few 

 scattered hill-tops left as islets, not one of them large enough to 

 nourish a glacier of sufficient extent to accumulate and deposit 

 Boulder-clay at the high levels which it now occupies. 



During a period of subsidence, as the area of the land became 

 more and more contracted, both snow-fields and glaciers would like- 

 wise diminish in extent ; glaciers would cease to ride over the minor 

 elevations, and would be restricted to narrower valleys ; some of the 

 steeper hill-sides would be uncovered, and, breaking up under the 

 influence of the frost, would shower down upon the surface of the 

 glaciers heaps of broken rocks and debris. This rubbish, eventually 

 reaching the ends of the glaciers, would be shot forward into the 

 sea. Supposing, therefore, that the Till had been deposited on the 

 sea-bottom during a period of subsidence, we ought to find this 

 deposit, as we approach its upper limits, becoming coarser and 

 earthier, and more and more charged with rough angular blocks and 

 debris, scratched stones being in the minority. But nothing of the 

 kind takes place. The Till at the very highest levels is identical in 

 structure and composition with the same deposit in the bottoms of 

 the valleys in central Scotland — the stones it contains are not less 

 well polished and striated, and rough unsmoothed angular blocks and 

 debris occur just as seldom. 



The only theory which seems to satisfy all the requirements of 

 our present knowledge is that so long ago advanced by Agassiz. 

 E very part of Scotland, with the possible exception of a few peaks 

 or tips of some of the loftier mountains, has certainly been buried 

 underneath snow and ice. This mer de glace must have levelled up 

 the valleys and occupied all the fiords, sounds, and shallow seas 

 around our island. Below this deep sea of ice hill-slopes were 

 ground and polished, valleys were deepened and smoothed, and the 

 wreck and rubbish resulting from all this work gathered unequally 

 below the ice, according as the direction and pressure of the superin- 

 cumbent mass determined. On steep slopes, where the motion of the 

 ice would be more rapid, the clay and stones would not readily collect, 

 but as the ice crept across the low ground deep accumulations of such 

 waste materials would begin to thicken below it. It is not, of course, 

 supposed that Boulder-clay continued to increase to an indefinite 

 thickness, nor that, having been once formed, it remained at rest 

 below the ice. If such had been the case, it would be difficult to 

 see how the ice could have had much denuding effect upon the sub- 

 jacent rocks ; for the moment that a layer of Till had been formed 



