284 PKocEEDiKas 01' THE aEOLoaiCAL SOCIETY. [May 12, 



lias been followed by more limited and local ice-action. Even on 

 the same rock-surface two intersecting sets of striae may be seen, 

 the older set quite independent of the present outline of the coun- 

 try, the newer clearly determined by the existing mountain-valleys 

 and glens. It is quite possible that, after the land emerged from 

 the sea, ice may have again formed in the mountain-corries and 

 flowed in vast rivers down the glens. There is, indeed, evidence that 

 this has occurred, in at least the higher portions of the district. 

 But we must ever be cpreful to avoid confusing these mere local 

 and partial glaciers, very limited both in size and influence, with 

 the far older and more universal ice-action of the first period. The 

 earlier glaciers can have had no connexion with the origin of the 

 lines ; the later may. 



When examining the lines on various occasions, I have ever been 

 on the search for some fact or facts that might serve as a criterion 

 of the truth or falsehood of the rival theories. At one time I 

 thought the character of the shingle composing them, and the mode 

 of its deposition, might serve this purpose. But I could find no 

 character of this kind at all satisfactory. The shingle on the shores 

 of our Highland fresh- water lakes and that on those of the inland 

 salt-water lochs, are too similar in most respects to admit of accu- 

 rate discrimination. At length one character did occur to me of a 

 testing and discriminating kind, one point in which the two views 

 were essentially different. Mr. Milne long ago pointed out the re- 

 markable fact that the three best-marked lines corresponded nearly 

 with cols or gorges between the hills, and showed that these gorges 

 must have formed the outlets for the fresh-water lakes to which he 

 ascribed the cutting out of the lines. Now of this there can be no 

 doubt. If from any cause the water of the Spean was prevented from 

 flowing off to the west till it rose to the height of the first or lower 

 line, it would form a lake flowing off to the Spey by the pass of 

 Maccoul; so also a barrier raising the waters of the Eoy to the 

 second line would cause them to overflow by Glen Glaister ; and 

 were this exit also shut, when they rose to the third or highest line 

 they would then escape at the very top of Glen Eoy into the Spey. 

 How far the levels of the respective lines correspond exactly with 

 these cols must be left to the Ordnance Survey to decide. The levels 

 are either very near, or perhaps the lines in some cases a little 

 higher; so that the supposed lakes would necessarily have overflowed 

 at the points mentioned. 



It is thus certain that, if the lines were formed by fresh-water 

 lakes, each of these passes must have been the exit of a river of very 

 considerable size and flowing in a narrow valley for a long period. 

 If, on the other hand, they are of marine origin, then these same 

 passes were sea-straits — narrow channels connecting one great bay 

 with another. Here then was a marked difference in the two theo- 

 ries, a matter of fact which existing phenomena might enable us 

 to decide. For this purpose I examined the various passes carefully, 

 and found that whilst in none of them was there the slightest trace 

 of an ancient river, in all there were distinct indications of the 



