of Edinburgh, Session 1870-71. 
401 
bed. But here, again, the objections are equally decisive. First , 
the raising of a river bed in this way seems to take place only when 
the current has reached some comparatively level part of its course, 
as in the Po or Nile. The Spey is remarkable for the steep incline 
of its bed. The Ordnance Survey * shows that for nearly 30 miles 
below Grantown it goes down more than 600 feet,—fully 20 feet a 
mile. The current is strong, the old terraces are high. The idea 
is not for a moment to be thought of that it could have acted as the 
sluggish rivers which silt up their beds. But, secondly , how did the 
river, after silting up its bed, and raising itself, come to change its 
action, and cut its way down ? Is any such case on record appli¬ 
cable to any river course as a whole ? If such a revolution of 
river action be exceptional, or if it be unknown in nature, we 
should surely not be warranted in applying it to the rivers of Scot¬ 
land generally at the period of the terraces. 
Thus the idea that the river bed had formerly been elevated is 
encompassed by difficulties. In whatever form the explanation is 
put, objections at once suggest themselves which would appear to 
be fatal. 
Turning to the other view, that the river had flowed on its pre¬ 
sent level, we find that the one great difficulty is the vast amount 
of water which would be needed to flood the channels up to the 
requisite height. Mr Prestwich, referring to the Somme and some 
English rivers, has calculated that it would require 500 times the 
present flow of the stream to form the 80 feet terrace.f When we 
look closely into the matter, however, this difficulty diminishes. 
The result of 500 : 1 is obtained by taking the present flow of the 
Somme at 800 square feet sectional area. That represents the 
river when not in flood. As the 80 feet terrace, however, is ad¬ 
mittedly the work of the old river when in flood, we must take the 
present Somme also in flood, and that is not 800 but 3000 square 
feet (Prestwich). + The effect of this first correction is to bring 
the 500 : 1 down to 133 : 1. But, further, when Mr Prestwich 
comes to put all the facts together, he estimates the old Somme at 
a little more than five times the present—16,000§ against 3000 of 
* As yet unpublished ; but these r^ults were obligingly communicated 
by Col. Sir H. James, F.R.S. 
+ Phil. Trans., vol. cliv., p. 265. 
+ Ibid., 292. g Ibid. 
