The author, after referring to the views of former observers, stated, that though he had proceeded to Glen Roy under a strong impression that Mr Darwin's marine theory afforded a solution of the question, he had felt himself constrained, after an examination of the valleys, to abandon that theory, and that he had satisfied himself that the shelves had been formed by lakes of fresh water. He referred to the proofs still existing of the mode in which these lakes had been discharged, and he described particularly the unequivocal traces of an old river course running from the head of Glen Glaster to Loch Laggan, with a delta at the level of shelf 4 on Loch Laggan side, formed by this ancient but now extinct river.