1863.] JAMIESON PARALLEL EOADS OF GLEN EOT. 257 



slightly — or about 1 foot in a mile — as we trace it eastward. I was 

 unfortunately unaware of the Ordnance-levellings when at Loch 

 Laggan and Makoul, otherwise I should have tested this poiut there, 

 but hope to be able to do so at some future time. Mr. Milne-Home, 

 however, says that, by barometrical measurement, the height of the 

 line is 21 feet above the highest point of the channel at Makoul, and 

 as the Ordnance-levelling indicates 851 feet for the height of the 

 watershed there, this would give (851 + 21) 872 feet for the altitude 

 of the line at Makoul. There is therefore some ground for suspecting 

 that this rise to the eastward extends to Makoul, and in a similar 

 proportion to the horizontal distance. The verification of this inter- 

 esting question, however, requires further investigation. Should a 

 rise to the eastward be actually established, one might speculate upon 

 its connexion with what I have hinted concerning the raised beaches 

 and shell-beds of Norway. 



§ 5. Objections to a Dam of Ice. 



The greatest difficulty that I find in supposing the Parallel 

 Roads to have been formed by glacier- dammed lakes arises from a 

 consideration of the depth of water the ice had to retain ; for it is 

 evident the moraines were too insignificant to have done much of the 

 duty. One might think the hydrostatic pressure of a column of 

 water some hundreds of feet high would have forced an escape 

 beneath the ice. If, however, the height or thickness of the glacier 

 were sufficiently in excess of the depth of water, I imagine there 

 would be pressure enough to keep it in. The difficulty may also be 

 lessened by supposing the grinding of the ice to have considerably 

 deepened the bottom of the Great Glen since the time of at least the 

 higher lines. 



Some low rocky eminences stretch across the mouth of Glen 

 Spean and Glen Gluoy, and may have blocked them up to some 

 extent before the gaps around them were ground out by the ice. 

 Our knowledge of glacier-action is but recently acquired, and has 

 been derived chiefly from a study of one region, namely Switzerland. 

 Until a country like Greenland or Spitsbergen has been examined, 

 our acquaintance with the geological effects of land-ice will be very 

 imperfect. I, therefore, do not think the objection I have stated is 

 sufficient to counterbalance the amount of evidence in favour of the 

 theory. 



§ 6. Central Asia during the Glacial Period. 



If I am right in my explanation of these Glen Roy lines, it is 

 clear that many stratified accumulations, presenting all the features 

 of aqueous deposits, might be heaped up in pools or lakes confined 

 by the ice in situations where, without this agency, we could not 

 fancy any body of fresh water to have existed. I have no doubt 

 this has been a fertile source of delusion in countries formerly over- 

 spread with glaciers. In such regions many of the supposed proofs 

 of the former presence of the sea far inland, and at great heights, 

 whether drawn from parallel roads, perched boulders, gravel ter- 



