[A] See January No., p. 61.
[B] Having enumerated the characteristic features of the glacial
phenomena in the preceding pages, I throw into this note some
explanation which may render my views of the parallel roads more
intelligible, not to interrupt again the exposition with details. It
would be desirable, however, that the reader should first make himself
thoroughly familiar with the localities concerned, before proceeding any
farther. I would therefore state here, that, in the wood-cut opposite,
G. R. indicates the valley of Glen Roy, with the three parallel roads
marked 1, 2, 3. Glen Spean is designated by G. S., and the river flowing
at its bottom by S. Loch Laggan, out of which the River Spean rises, is
marked L. G. indicates Glen Gloy, a little valley to the northwest of
Glen Roy, with a single terrace. Loch Treig is designated by T., Loch
Lochy by L. O., Loch Arkeig by A., and Moeldhu Hill by M., while E.
indicates Loch Eil. The Great Glen of Scotland, through which the
Caledonian Canal runs, extends in the direction of L. O. and E. The
position of Ben Nevis is designated by N. The dotted area between N. and
M. marks the place occupied by the great glacier of Ben Nevis, when it
extended as far as Moeldhu; while the close continuous lines in front of
Loch Treig indicate the direction of the glacial scratches left across
Glen Spean by the glacier of Loch Treig, when it extended as far as the
eastern termination of the two upper terraces. It ought to be
remembered, in this connection, that the bottom of the valley of the
Spean, as well as that of Glen Roy, is occupied by loose materials,
partly drift, that is, materials acted upon by glaciers, and partly
decomposed fragments of rocks brought down by the torrents, greatly
impeding the observation of the polished surfaces. The river-bed is cut
through this deposit, and here and there through the underlying rock.
Besides the parallel roads, there are also peculiar accumulations of
loose materials in Glen Roy and Glen Spean, more particularly connected
with the lowest terrace, which Mr. Darwin and Professor Jamieson have
shown to be little deltas formed during the existence of the lake of
Glen Roy at the bottom of the gullies intersecting the shelves of the
upper roads. The outlet for the water at the period during which the
second terrace was formed, not known when I visited Glen Roy, has been
discovered by Mr. Milne-Holme, and also observed by Professor Jamieson.
During the formation of the upper terrace, the waters escaped through
the westernmost tributary of the River Spey, in the direction of the
northeast corner of the wood-cut, and during that of the lowest terrace,
at the eastern end of Loch Laggan, also through the valley of the Spey.
The state of preservation of the parallel roads is such as to prove that
no disturbance of any importance can have taken place in the country
since they were formed. Far from believing, therefore, that these
remarkable shelves are ancient sea-beaches, I am prepared to maintain,
that, had the area occupied by them been submerged only for a few days,
under an ocean rising and falling for several feet with every tide, no
vestige would have been left of their former existence.
[C] The wood-cut on p. 730 is a reproduction of the little map
accompanying a paper of mine upon "The Glacial Theory and its Recent
Progress," printed in the "Edinburgh New Philosophical Journal" for
October, 1842. I might have greatly improved the topography, and
represented more accurately the details of the phenomenon, by availing
myself of the much larger and very minute map recently published by
Professor Thomas F. Jamieson, of Aberdeen; but I thought it advisable to
leave my first sketch as I presented it twenty-two years ago, in order
to show that Sir Charles Lyell is mistaken in ascribing (see "Antiquity
of Man," pp. 260, 261) the discovery of the glacier of Loch Treig to
Professor Jamieson. A comparison of his statements with mine will show
that the solution of the problem offered by him is identical with that
proposed by me, as he himself candidly admits ("Quarterly Journal of the
Geological Society" for August, 1863, p. 239). I have only one fault to
find with his observations, and, as I have never revisited the locality
since, this remark may satisfy him that my examination of its features
was not so hurried as he supposes. Professor Jamieson confounds the
effects of two distinct glaciers moving in different valleys as the
action of one and the same glacier. In my paper, it is true, I made no
allusion to the great glacier of Glen Spean, the existence of which I
had recognized along the river from Loch Laggan nearly to the Caledonian
Canal. I publish my observations upon this great central glacier for the
first time in the present article, having omitted them in my
contributions upon this subject to the scientific periodicals of the day
simply because I thought best not to complicate my exposition of the
facts concerning the parallel roads by considerations foreign to their
origin, convinced as I was, from the manner in which the glacial theory
was then received, that they would not be understood, and still less
admitted. But now that all the geologists of Great Britain seem to have
given their adhesion to it, I may be permitted to state that I already
knew then, what Professor Jamieson has overlooked in his latest paper,
that a separate glacier had occupied the valley of the Spean _prior_ to
the formation of the parallel roads, and that at that time the glacier
of Loch Treig was only a lateral tributary of the same, just as the
glacier of the Thierberg is a tributary of the glacier of the Aar. It
was not until the Glen Spean glacier had retreated to the hills east of
Loch Laggan that the glacier of Loch Treig could form a barrier across
Glen Spean, and thus dam the waters in Glen Roy which produced the
parallel roads. The marks left by the great Glen Spean glacier in the
valley are mistaken by Professor Jamieson for indications, that, in its
greatest extension, the glacier of Loch Treig not only advanced across
Glen Spean, but divided into two branches, one moving westward down Glen
Spean, the other eastward up Glen Spean, as far as Loch Laggan. Any one
sufficiently familiar with existing glaciers to compare their action
with the phenomena referred to above will at once see the impossibility
of such a course for any glacier coming down from Loch Treig. At the
time the Grampians had become a separate centre of glacial action a
great glacier must have moved down, towards the Caledonian Canal,
through Glen Spean, receiving as tributaries lateral glaciers not only
from Loch Treig and Glen Roy, but also from all the other minor lateral
valleys emptying into Glen Spean, the largest of which must have come
from the range of Ben Nevis,--just as the great glacier of the valley of
the Rhone once received as tributaries all the glaciers coming down into
that valley from the southern slope of the Bernese Oberland, and from
the northern slope of the Valesian Alps, and at one time also from the
eastern slopes of the range of Mont Blanc. And when the large glacier
occupying the lower, and therefore warmer, level gradually disappeared
and retreated far away to levels where it could maintain itself against
the effect of a returning milder climate, the opening spring of our era,
as we may call it, the lateral glaciers, arising from the nearer high
grounds, could extend across the valleys, but not before.