112 MR DAVID MILNE HOME: MEMOIR ON THE 
then they have been cut through by the Mouse water, and the lake has thus 
been drained. But its séée is still visible” (p. 312). 
Looking to these passages, and others, in Professor GEIKIE’s writings, it 
is difficult to understand how he should have given it as his opinion, that 
had detrital barriers existed in Glen Roy to dam back the lakes, they 
“would undoubtedly have been able to maintain themselves, and be still 
extant.” 
There would, of course, be a greater probability of removal by rain and 
streams, if the detritus forming the blockage consisted of soft sandy mud. Now, 
as previously noticed, at and near the places where the Glen Roy and Glen 
Collarig blockages occurred, there is great abundance of such kind of detritus. 
To that circumstance is owing the great breadth of Shelf 3 on the N.E. shoulder 
of Craig Dhu, extending to 100 yards, as indicated even on the Ord- 
nance 6-inch Map. To show how easily detritus of a soft muddy character 
may be eroded by rain and small burns, reference may be made to 
the circumstance that Shelf 4, which must have existed round the west 
shoulder of Craig Dhu and Meall Dherry, is for more than a mile not traceable. 
It, however, must have at one time existed there. As the lake reached to this 
part of the valley, a beach line must have been formed here as elsewhere, and 
the only explanation is, that the beach line at this place was washed away by 
the stream descending the sides of the hills. 
The same remark applies to the discontinuance of Shelf 4 on the N.E. side 
of Ben Chlinaig. It will be seen from the map of Lochaber, that for more than 
a mile, the shelf is not traceable on this hill-side. But it must at one time 
have existed there also. The materials which composed it have been removed 
by streams and rain. 
T have in the foregoing remarks assumed the correctness of Dr TyNDALL’s 
statement, that not a trace is to be now seen of the detrital barrier by which 
the lake in Glen Roy was kept up at its two successive levels. But regarding 
the correctness of this assumption, some doubt exists. In my previous Memoir 
(p. 620) I mention that, at the places where Shelves 2 and 3 terminate in 
Glen Roy, there are banks of detritus in a direction transverse to the valley, 
which greatly resemble the remnants of a barrier. 
I would only add, that I observe in Dr TynpDALL’s lecture, with satisfaction, 
a full admission of the enormous extent of detritus abounding in the Lochaber 
district. He speaks of the “vast masses of detritus in certain portions of Glen 
Spean” (p. 5); and of “the friable drift over-speading the mountains” (p. 4). 
He, moreover, explains (p. 2) that “the Parallel Roads are terraces formed m 
the yielding drift, which here covers the slopes of the mountains.” 
Having made these admissions, Dr TYNDALL must at all events allow that 
the materials were ample for furming the required blockages. 

