i84i 1880] GLEN ROY 185 



Mr. Milne's discovery of the outlet to the intermediate shelf Letter 522 

 at Glen Glaster *), Mr. Milne gives only half of my explana- 

 tion ; he alludes to (and disputes) the smoothing and silting- 

 up action, which I still believe in. I state : If we consider what 

 must take place during the gradual rise of a group of islands, 

 we shall have the currents endeavouring to cut down and 

 deepen some shallow parts in the channels as they are suc- 

 cessively brought near the surface, but tending from the 

 opposition of tides to choke up others with littoral deposits. 

 During a long interval of rest, from the length of time allowed 

 to the above processes, the tendency would often prove effect- 

 ive, both in forming, by accumulation of matter, isthmuses, 

 and in keeping open channels. Hence such isthmuses and 

 channels just kept open would oftener be formed at the level 

 which the waters held at the interval of rest, than at any 

 other (p. 65). I look at the Pass of Mukkul (21 feet deep, 

 Milne) as a channel just kept open, and the head of Glen Roy 

 (where there is a great bay silted up) and of Kilfinnin (at both 

 which places there are level-topped mounds of detritus above 

 the level of the terraces) as instances of channels filled up 

 at the stationary levels. I have long thought it a probable 

 conjecture that when a rising surface becomes stationary it 

 becomes so, not at once, but by the movements first becoming 

 very slow ; this would greatly favour the cutting down many 

 gaps in the mountains to the level of the stationary periods. 



Glacier Theory. If a glacialist admitted that the sea, 

 before the formation of the terraces, covered the country 

 (which would account for land-straits above level of terraces), 

 and that the land gradually emerged, and if he supposed his 

 lakes were banked by ice alone, he would make out, in my 

 opinion, the best case against the marine origin of the terraces. 

 From the scattered boulders and till, you and I must look at 

 it as certain that the sea did cover the whole country, and 

 I abide quite by my arguments from the buttresses, etc., 

 that water of some kind receded slowly from the valleys of 

 Lochaber (I presume Mr. Milne admits this). Now, I do not 

 believe in the ice-lake theory, from the following weak but 

 accumulating reasons : because, ist, the receding water must 

 have been that of a lake in Glen Spean, and of the sea in the 



1 See Letter 521, note 2, p. 181. 



