184 GEOLOGY [CHAP. IX 



Letter 522 This appears to me the best and newest part of the paper. 

 If Sir James Clark 1 would like to attend to any particular 

 points, direct his attention to this : especially to follow Glen 

 Glaster from Glen Roy to L. Laggan. Mr. Milne describes 

 this as an old and great river-course with a fall of 212 feet. 

 He states that the rocks are smooth on upper face and rough 

 on lower, but he does not mention whether this character 

 prevails throughout the whole 212 vertical feet a most im- 

 portant consideration ; nor does he state whether these rocks 

 are polished or scratched, as might have happened even to a 

 considerable depth beneath the water (Mem. great icebergs in 

 narrow fiords of T. del Fuego 2 ) by the action of icebergs, for 

 that icebergs transported boulders on to terraces, I have no 

 doubt. Mr, Milne's description of the outlets of his lake 

 sound to me more like tidal channels, nor does he give any 

 arguments how such are to be distinguished from old river- 

 courses. I cannot believe in the body of fresh water which 

 must, on the lake theory, have flowed out of them. At the 

 Pass of Mukkul he states that the outlet is 70 feet wide and 

 the rocky bottom 21 feet below the level of the shelf, and that 

 the gorge expands to the eastwards into a broad channel of 

 several hundred yards in width, divided in the middle by 

 what has formerly been a rocky islet, against which the waters 

 of this large river had chafed in issuing from the pass. We 

 know the size of the river at the present day which would 

 flow out through this pass, and it seems to me (and in the 

 other given cases) to be as inadequate ; the whole seems to 

 me far easier explained by a tideway than by a formerly 

 more humid climate. 



With respect to the very remarkable coincidence between 

 the shelves and the outlets (rendered more remarkable by 



1 Sir James Clark (1788-1870) was for some years a medical officer 

 in the Navy : he afterwards practised in Rome till he moved to London 

 in 1826. On the accession of Queen Victoria he was made Physician in 

 Ordinary and received a baronetcy ; he was elected into the Royal 

 Society in 1832. (Diet. Nat. Biog., 1857 ; article by Dr. Norman 

 Moore.) 



2 In the Voyage of the Beagle a description is given of the falling of 

 great masses of ice from the icy cliffs of the glaciers with a crash that 

 " reverberates like the broadside of a man-of-war, through the lonely 

 channels " which intersect the coast-line of Tierra del Fuego. Loc. cit. 

 p. 246. 



