Note C — THE PARALLEL ROADS OF GLEN ROY, IN SCOTLAND.

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.