•.• 349* 



and the valley of the Spey. The debacle coming' from the NW. 

 would fill these glens, and flow over the summit levels. As soon 

 as it subsided below that of Glen Gluoy, that glen would be in 

 the condition of a lake or arm of the sea, and defended from the 

 great tunmlt of waters by the mountain ridge betwixt it and the 

 Great Glen, while the agitation would be sufficient quickly to 

 produce the highest of the shelves. When the waters subsided be- 

 low the summit level of Glen Roy, then the highest shelf in that 

 glen would begin to be formed. As the subsidences would be ir- 

 regular, in consequence of the agitation of the waters, and the in- 

 fluence of the varied surface of the land, and as also the tides would 

 contribute in a considerable degree to this irregularity, Sir George 

 conceived a sudden subsidence, and a sudden stoppage for a time, 

 amply sufficient for the formation of the imperfect terraces or 

 shelves, the sections of which clearly indicate something very dif- 

 ferent from the comparatively tranquil operations of the sea on 

 the coast. The lowest shelf appears to have formed on the subsi- 

 dence of the water below the summit level betwixt Glen Spean 

 and the valley of the Spey. The shelves disappear at the points 

 where the agitation of the waters may be supposed to have been 

 too great for their formation, on account of vicinity to the Great 

 Glen ; and, consequently, we may expect the diluvium to assume 

 the appeurar.ce in that locality which it exhibits in many others, 

 and modified by the shape of the surface. This modification is 

 beautifully exhibited whenever the effects of the land on the move- 

 ment of the waters is taken into account, and which it has never 

 been sufficiently, when the theory of a debacle has been discussed. 

 Sir George admitted fully the probability that masses of ice, 

 brought by the debacle, may have rested at the openings of these 

 glens, and acted as barriers to a certain extent ; but this, he consi- 

 dered, docs not affect his general theory, while it rather strengthens 

 it. The erratic blocks found on the shelves may have been depo- 

 sited by ice ; and if rounded pebbles, brought in evidence by Mr 

 Darwin against Sir George's theory, could not have been produced 

 by such a vast torrent, the great abundance of ready-made pebbles 

 to be found in the masses of conglomerate over which the flood 

 had to pass, would sufficiently remove the objection. Sir George 

 expressed himself as by no means anxious about the fate of his 

 theory, because a philosophical examination of it, if condemnatory, 

 would remove obstacles in the way of arriving- at truth ; and if 

 laudatory, would load to a correct explanation of facts still in a 

 somewhat anomalous state. 



