OF THE PARALLEL ROADS OF LOCHABER. 
675 
It would, as he observes, “ require us to suppose that the ice was much more de¬ 
veloped in Ben Nevis and the region to the west of the Caledonian Canal than in the 
hilly district around the sources of the Spey.* .... Otherwise they (the glaciers), 
would have occupied the watersheds or cols at the time the terraces show they were 
clear of ice.” 
The reason he suggests for conditions so anomalous, is a difference in the precipita¬ 
tion of snow such as now exists in the rainfall of the same districts. He refers to 
the mean annual fall at Fort William of 86 inches, and at Loch Nevis of 82 inches,! 
whereas at Laggan it is only 46 inches. But Laggan is as far from the hills in 
question as is Fort William, and more recent observations given by Mr. Symonds 
(‘British Rainfall,’ 1876-7) show that the rainfall at the Bridge of Roy is in fact 
62 inches annually, and that that place comes within the line of above 50 and under 
75 inches fall. 
Mr. Jamieson’s view is, however, supported by the authority of Professor Tyndall,! 
who considers that the position and greater height of the mountains south of Glen 
Spean caused them to intercept the moisture carried by the Atlantic winds which 
would therefore have reached the hills north of that glen in a state of greater dryness 
and warmth. 
But the difference alone between the height of the hills from the west coast at Mull 
to Ben Nevis and of the hills eastward of Ben Nevis, is, on the whole, with the 
single exception of this and one adjacent mountain, so small, that the effect of greater 
condensation could have been only of a very limited local character, apart from the 
general decrease which everywhere obtains in proceeding from the west to the east 
coast.§ That this general condition contributed however to a much larger development 
of the ice-sheet on the west coast than on the east is tolerably certain. Its dimen¬ 
sions in Skye and on the north-west coast seem to have much exceeded anything in 
the more eastern counties. It is also probable that the formidable block of ice on the 
west coast was the cause of the eastward movement of a large portion of the ice-sheet 
and afterwards, for a time, of the surface waters. But this is part of a general 
phenomenon to which the Ben Nevis range is subordinate, and could hardly have 
been productive of any extreme differences in such closely similar and conterminous 
districts. 
It must further be borne in mind that the land probably stood higher, and that the 
coast line was considerably farther distant from Ben Nevis at the period referred to 
than at present (see § 11). Still the relative height of the hills would have been much 
the same then as now, when with the exception of Ben Nevis which is 4,406 feet, and 
* Quart. Joum. Geol. Soc., vol. xxx., p. 335, 1874. 
f Glen Quoich, where the fall is 102 inches, is also mentioned, but this lies in another direction 
20 miles N.W. from Glen Spean. 
X “On the Parallel Roads of Glen Roy,” Proc. Roy. Inst, of Gt. Britain for June, 1876, p. 11. 
§ Glen Roy lies N.N.E. of the Ben Nevis range, so that westerly and W.S.W. winds would reach it 
without passing over those mountains. 
