
PARALLEL ROADS OF LOCHABER. Ltt 
tion of how the lake barriers had been heaped up, but how they might have 
been ruptured and broken down. 
Dr TynpAtt has unfortunately made another mistake in his representation 
of Sir THomas LAUDER’s views. He says, that to explain the formation of the 
middle Parallel Road, “Sir THomas invoked a new agency,” viz., “a halt” in 
the “breaking down or waste of his dam.” I take leave to say that Sir 
THomaAs LAUDER invoked no agency whatever to explain the formation of the 
middle Parallel Road. The lake subsided by several steps from the upper- 
most to the middle Parallel Road, and stood there long enough to form 
that middle Parallel Road. The barrier held firm at that point long 
enough to allow of a strong beach line being formed. What new agency Dr 
TYNDALL can allude to, as having been invoked or invented by Sir THomas 
Lauper, I cannot imagine. 
The only other remark made ae Dr TynDALL by way of objecting to the 
detrital theory is, that “no barriers of detritus could have existed, without leaving 
traces behind them. But (he says) there is no trace left. The two highest Parallel 
Roads stop abruptly at different points near the mouth of Glen Roy. No 
remnant of the barrier against which they abutted is to be seen;” and he 
quotes an opinion, said to have been given and “insisted on by Professor 
GEIKIE, that barriers of detritus would undoubtedly have been able to maintain 
themselves, had they ever been there” (pp. 6 and 12.) 
On this special ground, and, so far as I can discover, on this ground alone, 
Dr TynpAtt told his audience, that they “ may with safety dismiss the detrital 
barrier theory.” 
With regard to the opinion ascribed to Professor GEIkiz, I cannot help 
thinking, that the views of the learned Professor must have been misappre- 
hended. No geologist has shown better than Professor GEIkIEe himself, the 
enormous denudation of detritus effected by such agents as some of those 
suggested for the erosion and removal of the Lochaber blockages. 
In Professor GxErkiz’s popular work, entitled ‘Scenery and Geology of 
Scotland,” the following passage occurs :— 
“ Let any one stand on the ice-worn barrier of rock between Loch Ness and 
Loch Oich. He will see there, that even on the supposition of an open fissure, 
the deep concavity of the (Great) Glen at this point must be due to denudation.” 
“The very arrangement of the roeks is enough to prove that the hollow has 
been worn out by the agencies of nature. The glen at Fort Augustus must be 
due mainly to denudation” (p. 178). 
Farther, to show the effects of denudation on detritus, the Professor refers 
to the removal of lake barriers by the same agencies. He mentions that near 
Carstairs “the Kaimes stretch across the mouth of a broad valley, where they 
must at one time have dammed back the drainage, so as to form a lake. Since 
