In the incidental notices of these islands, given in Chapter II., the narrators sometimes describe them as having been built on a framework of oak, as at Lochrutton, Loch Kinder, etc. At other times the only evidence of a crannog consists in fishing up oak beams from the bottom of a lake, or their discovery under its surface inextricably mortised into others, as at Loch Lomond, Lochmaben, Loch Lochy, etc., while in drained lakes and morasses the chief indications are the tops of upright piles surrounding a flat mound. But all these accounts, as well as the more recent notices of crannogs, are characterised by two prominent structural features, viz., (1) upright piles in the form of one or more circles; and (2) the remains of flat beams containing large square-cut holes at their extremities. The quotations from the Irish writers, given at pp. 6-11, will also show that the same structural features characterise many of the descriptive notices of the crannogs of Ireland. Dr. Stuart also draws particular attention to similar beams at Dowalton, some of which were of "great size and length (one of them 12 feet long), with three mortised holes in the length, 7 inches square." These indications, as well as a careful study of the structural details recorded in my reports of the explorations made at Lochlee, Lochspouts, and Buston, have led me to believe that all the wooden islands were constructed after one uniform plan, and that this plan was actually the outcome of the highest mechanical principles that the circumstances could admit of. Indeed, I am prepared to maintain that were the same problem submitted to modern engineers, they could make no improvement either on the principle or mechanism displayed in these singular structures. Let me therefore, in the first place, note the conditions that called forth such high mechanical and engineering qualities. For defence and protection, which, I presume, no one will doubt were the primary objects of these islands, a small mossy lake, with its margin overgrown with reeds and grasses, and situated in a secluded locality amidst the thick meshes of the primeval forests of those days, would present the most desirable topographical conditions. Having fixed on such a locality, the next consideration would be the selection of materials for building the island. In a lake containing the soft and yielding sediment due to decomposed vegetable matter, it is manifest that any heavy substances, as stones and earth, would be totally inadmissible owing to their weight, so that solid logs of wood, provided there was an abundant supply at hand, would be the best and cheapest material that could be used. To construct in 10 or 12 feet of water, virtually floating over an unfathomable quagmire, a solid compact island, with a circular area of 100 feet or more, and capable of enduring for centuries as a retreat for men and animals, would, I daresay, be the means of eliciting from many an engineer of the present day a more