_Mode of Construction of._--Having decided on the position, the _crannog_ builder set to work by driving stakes into the bottom of the lake in a circle of from sixty to eighty feet in diameter, a considerable length of the stake sometimes projecting above the water; these were in many instances joined together by horizontal beams, the interior filled up by branches of trees, stones, gravel, earth, and bracken. Often an inner row (or more than one) of piling is found about five feet distant from the outer, and piles are driven in various parts of the interior, either to consolidate the mass or to act as stays for the walls of the dwelling. Next were placed one or two layers of round logs, cut into lengths of about six feet, generally mortised into the upright piles, kept in position by layers of stone, clay, and gravel. In some cases, where the foundation was soft, the superincumbent layers of timber were of great depth; in other cases, where the bottom of the lake was firm, the platform of timber was confined to a portion of the island. The side most affected by the action of the water was frequently strengthened by rows of piles, sometimes five or six deep, as well as by a breakwater of stones.[43] On the foundation, when raised sufficiently above the water, the dwelling was erected; the hearth was in the centre of the island, for in almost every case a collection of flag-stones has been discovered in the interior of the enclosure, bearing on them marks of fire. At times several hearths occur. In some instances there are indications of these structures having had additions made to their height at various times, either to keep pace with the chronic rising of the level of the lough; or, taking into consideration the compressible nature of the component parts of the foundation, the island may have required increased elevation, owing to the effect of natural subsidence. The enormous amount of wood employed in the formation of a crannog, despite the mass of stones and other material with which it was laden, must have had at first a buoyant tendency, but according as the wood became completely saturated with water, consolidation of a more or less pronounced character would, in the course of time, become observable. The stones deposited over the lower strata of fern, bracken, and branches, were indubitably used to compress and solidify the substructure, as likewise were the stones on the upper strata used for a similar purpose. Considerable ingenuity was displayed in the formation of these island homes, which were frequently constructed in a depth of twelve to fourteen feet of water; and, apart from having served in their day as secure retreats for large numbers of persons, they have proved their durability by resisting successfully the ravages of time, which may be reckoned by centuries.