
D. MILNE HOME ON THE PARALLEL ROADS OF LOCHABER. 599 
sists of a rocky ridge formed by the upturned edges of Mica Slate strata. These 
strata have been worn down apparently by water flowing over them from the 
Glen Roy side, their smooth faces being on that side, their rough faces on the 
side next Glen Spean. On the shelf itself there are multitudes of large well 
washed pebles and boulders. When the lake stood at the level of the shelf, 
discharging at Mukkoul, the water then occupying Glen Roy would flow 
towards Mukkoul, and one of the passages would be the narrow and shallow 
strait just referred to. 
In Glen Roy, and at many other places, the middle shelf is seen to have 
been deeply cut through by burns, whose sides expose the material of the old 
beach and of the lake bottom. Thus at Dalrioch and on the N. E. shoulder of 
Craigh-Dhu, the old lake bottom consists of fine clay, or mud, horizontally 
stratified and laminated,—evidence of the stillness and depth of the water in 
which the sediment had been deposited. At the spot last mentioned, there are 
cliffs of sandy mud from 40 to 60 feet high. These beds of mud are occasionally 
covered by beds of stratified sand. 
In Glen Collarig, the lowest shelf on Bohuntine Hill is crossed and cut, 
through by a burn. The material there also consists of sandy mud with small 
boulders and pebbles. 
At Inverlair and Fersit, (near Loch Treig), there are large portions of the 
old bottom of the lake still extant, consisting of gravel cliffs from 50 to 60 feet 
in height. Great beds of sand occasionally occur in these gravel deposits. 
Fig. 1, page 4, is an attempt to show this old lake bottom cut through by 
streams from the adjoining hill. 
These facts supplied some of the grounds on which the lake theory rested. 
Other arguments will be noticed, when I refer more particularly to the sea 
theory. 
Meanwhile, I may allude to another fact recently ascertained which greatly 
strengthens the Lake hypothesis. 
Our colleague the Revd. Mr Brown has in one of the shelves (viz., the 
lowest of the three in Glen Roy, Shelf 4) discovered fresh water Diatoms. 
They could not have existed where he found them if the shelves were marine 
(“ Roy. Soc. Proceedings,” 2d March). . 
IV. The next point for consideration is by what means the lakes were 
kept up to the height of the shelves, in the absence of any appearance at 
present of Barrier or Blockage. 
In the lower part of Glen Roy the bottom of the valley is about 800 feet 
below the highest shelf, and the valley is about a mile wide. 
How then were the lakes kept in ? 
1. To understand clearly this part of the problem, it is proper to observe 
VOL. XXVII. PART Iv. | ray 
