"As a consequence, the only Welsh ice in position to obstruct the onward march of the invader would be such trifling valley-glaciers as could form on the western slopes of Snowdon itself. "The peak of Snowdon is 3,570 feet above sea-level, and Arenig Mawr, 2,817 feet high, is eighteen miles to the eastward, and a broad, deep valley with unobstructed access to Cardigan Bay intervenes; so, if any ice from the central mass made its way over the Snowdonian range, it performed a much more surprising feat than that involved in the ascent of Moel Tryfaen from the westward. "The profile shows in diagrammatic form the probable relations of the foreign to the native ice at the time when the Moel Tryfaen deposits were laid down. "From what has been said regarding the great glaciers, it would seem that ice advanced upon the land from the seaward in several parts of the coast of England, Wales, and the Isle of Man. Now, it is in precisely those parts of the country, and those alone, that the remains of marine animals occur in the glacial deposits. If the dispersal of the shells found in the drift had been effected by the means I have suggested, it would follow, as an inevitable consequence, that wherever shells occur there should also be boulders which have been brought from beyond the sea. This I find to be the case, and in two instances the discovery of shells was preliminary to the extension of the boundaries of the known distribution of boulders of trans-marine origin. "The officers of the Geological Survey some years ago observed the occurrence of 'obscure fragments of marine shells' in a deposit at Whalley, Lancashire, in which they could find only local rocks. One case such as this would be fatal to the theory of the _remanié_ origin of the shells, but on visiting the section with Mr. W. A. Downham, I found, amongst the very few stones which occurred in the shell-bearing sand at the spot indicated, two well-marked examples of Cumbrian volcanic rocks, and, at a little distance, large boulders of Scottish granites.