110 MR DAVID MILNE HOME: MEMOIR ON THE 
Before explaining to his audience the grounds on which he supported 
the glacier theory, Dr TyNnpDALL endeavoured to combat the only theory 
opposed to it, viz., the theory of detrital blockages, first suggested by the late 
Sir TaHomas Dick LAuDER. 
My own share in this question Dr TYNDALL was well aware of ; for he refers 
to the two Memoirs which I read in this Society; but he gave no statement 
whatever of the facts and arguments advanced in these Memoirs. He went 
back to Sir THomas Dick LAUDER’s paper, 59 years old, for an exposition of the 
grounds on which the detrital theory was maintained, 
I am sorry to have to add that, in explaining to his hearers Sir THomas 
LAUDER’Ss views, Dr TYNDALL inadvertently gave a version of these views 
not strictly correct. 
The passage in Dr TynDALL’s lecture to which I refer, is as follows:— 
“There are at the present moment vast masses of detritus in certain por- 
tions of Glen Spean. Out of such detritus, Sir Tuomas LAupDER imagined his 
barriers to have been formed. By some unknown convulsion, the detritus had 
been heaped up.” 
Now, I affirm that Sir THomas LAUDER never supposed that the detrital 
barriers suggested by him had been heaped up, and still less that any convulsion 
effected that object. Nor has any supporter of the detrital theory entertained 
such an unlikely, not to say absurd, idea. The barriers which Sir THomas 
LAUDER supposed had kept in the lakes, consisted (to use his own words) of the 
“large depositions of alluvial clay, sand, rounded pebbles, and gravel, which 
present themselves everywhere, and more particularly towards the mouths of 
the different valleys.” 
Sir THomas considered that the “ alluvial depositions,” as he termed them, 
which overspread the hills and filled the valleys, supplied the required blockage. 
He did not imagine that to form a blockage, the detritus had been heaped up, 
either by a convulsion or by any special agency. The detritus existing 
naturally in the district, in his opinion, constituted the blockage. 
The only difficulty which Sir THomas Lauper had, was in regard to the 
removal of the barriers. He himself states, in regard to these, that it was 
“much easier to suppose the existence of them (the barriers), than to devise the 
means which operated in their removal” (page 51). | 
With regard to this last question, he referred to the Great Glen now 
occupied by the Caledonian Canal, as being probably an “‘immense rent pro- 
duced by some extraordinary convulsion;” and he suggested that the convul- 
sions attending the creation of this rent would so rive and shatter the country 
as to affect the lake barriers, and particularly those of Glen Gluoy and Glen 
Spean (page 56). 
When Sir THomas LAUDER referred to a convulsion, it was not in explana- 
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