346 Mr Milne on the Parallel Boads of Lochaber. 



lation of matter from opposition of tides, it is not in situations like 

 the heads of glens, which narrow to a point, and at that point are 

 separated by a small neck of land, — it is where there is space for a 

 considerable current on each side of the strait. 



For these reasons I consider that Mr Darwin's explanation of the 

 coincidence of the shelves with the water-sheds before described, is 

 quite inadmissible. 



2. The second serious objection to Mr Darwin's theory arises 

 from the fact, that the shelves in the different glens are not coinci- 

 dent in level. If they had been formed by arms of the sea, as the 

 land rose out of it, the sea should have formed lines in all the valleys 

 which it entered, at precisely the same levels. But neither of the 

 Glen Gluoy shelves is to be seen in any of the other valleys. So 

 also the No, 2, or highest shelf of Glen Koy, and the next lowest, 

 or No. 3, do not occur in the lower part of that glen, or in the ad- 

 joining valleys of Glen Glaster, Glen Spean, and Glen Treig. 



Mr Darwin attempts to explain one, but one only, of these cir- 

 cumstances, viz., the difference of level between No. 1 and No. 2 

 shelves, by a theory of very questionable soundness. He says, that 

 the tide in Glen Gluoy may have risen 20 feet higher at the head 

 of the estuary, than at the head of Glen Turret, It would be 

 necessary that it should rise 29 feet higher. But if this were the 

 case, then the shelves, at all events, in Glen Gluoy, would not be 

 horizontal or nearly so ; — they would have sloped upwards towards 

 the head of Glen Gluoy, by 29 feet in the course of 6 or 7 miles, — ■ 

 the length of the glen. ,But this would be inconsistent with the 

 great and well-established fact so characteristic of these Lochaber 

 shelves ; and moreover, though the beach-lines at the heads of the 

 two glens might not be exactly coincident in level there, they ought, 

 at all events, to be so at the mouths of the glens where the supposed 

 arms of the sea joined the main body of the ocean, — which is not 

 pretended. 



This theory, however, would explain merely the non-appearance 

 of shelf 1 in Glen Roy. The non-appearance of all the others is ac- 

 counted for by Mr Darwin, simply by supposing that something or 

 other had prevented them being marked in the other glens. 



In support of this view, Mr Darwin refers to two intermediate 

 shelves which are faintly traceable on Tombhran and elsewhere, in 

 order to shew that the water did produce marks at some places, and 

 not at others. But, from the faintness of those intermediate lines, 

 it is manifest that the water had stood at their level for a much 

 shorter period than at the levels of the principal shelves ; and, there- 

 fore, no fair inference can be drawn from the former applicable to 

 the latter. 



3. These considerations suggest, however, a separate and even a 

 more serious objection. Not only should the sea have made mark- 

 ings at the same levels in all the Glens of Lochaber, but it should 



