There are phenomena in Nature which give the clue to so many of its mysteries that their correct interpretation leads at once to the broadest generalizations and to the rapid advance of science in new directions. The explanation of one very local and limited problem may clear up many collateral ones, since its solution includes the answer to a whole set of kindred inquiries. The "parallel roads" of Glen Roy offer such a problem. For half a century they have been the subject of patient investigation and the boldest speculation. To them natural philosophers have returned again and again to test their theories, and until they are fully understood no steady or permanent advance can be made in the various views which they have suggested to different observers. The theory of the formation of lakes by barriers, presented by McCulloch and Sir T. Lauder-Dick, that of continental upheavals and subsidences, advocated by Sir Charles Lyell and Charles Darwin, that of inundations by great floods, maintained by Professor H. D. Rogers and Sir George Mackenzie, that of glacial action, brought forward by myself, have been duly discussed with reference to this difficult case; all have found their advocates, all have met with warm opposition, and the matter still remains a mooted point; but the one of all these theories which shall stand the test of time and repeated examination and be eventually accepted will explain many a problem besides the one it was meant to solve, and lead to farther progress in other directions. I propose here to reconsider the facts of the case, and to present anew my own explanation of them, now more than twenty years old, but which I have never had an opportunity of publishing in detail under a popular form, though it appeared in the scientific journals of the day. Before considering, however, the phenomena of Glen Roy, or the special glacial areas scattered over Scotland and the other British Isles, let us see what general evidence we have that glaciers ever existed at all in that realm. The reader will pardon me, if, at the risk of repetition, I sum up here the indications which, from our knowledge of glaciers as they at present exist, must be admitted, wherever they are found, as proof of their former existence. Such a summary may serve also as a guide to those who would look for glacial traces where they have not hitherto been sought.