The turf-cutter in Ireland finds that “usually the roots and trunks of the trees under the peat, or in the lowest strata, are principally those of the oak and yew, as if prior to the growth of the peat the low country was a vast forest of these trees. It would appear that subsequently mosses and other peat-producing plants began to grow and flourish, until eventually they stopped the drainage, and formed an envelope of peat, thus killing the trees, which one by one toppled over and were buried in the succeeding growth of peat. After the disappearance of the major portion of the oak trees, the bogs, year by year, gradually increased in depth, until apparently suddenly, for some as yet unexplained cause, their growth ceased, and on their surface forests, principally of deals, sprang up.”[8] Thus we see that since the glacial period there have been great changes in the aspect and the surface of Ireland: first, the great oak forest age; then an age in which was an active growth of peat; thirdly, a period when forests of deal sprang up; fourthly, again a period of luxuriant peat growth. The remains of human handicraft, in the form of log-houses or lake-dwellings, have been found buried under each and all these peat growths: a depth of 25 feet had overgrown the log-house discovered at Inver, Co. Donegal, and on the floor-level, outside the building, were traces of the corkers of the great oak forest age. It is practically impossible to estimate the rate at which a bog grows: if there be a fall, and consequent drainage, it will increase but little, whereas an undrained bog augments with considerable rapidity: so many contingencies are thus introduced as practically to invalidate in a great degree calculations regarding the growth of peat over prehistoric or other remains. G. H. Kinahan has estimated that in undisturbed conditions each year’s growth, represented by a layer or lamina somewhat resembling the markings on a forest tree, would average one hundred laminæ to the foot in white or surface turf, two to three hundred to the foot in brown turf, and six to eight hundred to the foot in black turf, so that the accumulation of 25 feet above the log-house at Inver, according to this painstaking calculation, would represent an age of startling remoteness. _Lakes._--Ireland was a land of lakes as well as of forests, for the white-shell marl, which forms the substratum of peat bog in low-lying situations, was formerly covered by water, till gradually displaced by the encroachments of the surrounding bog. Many of the smiling districts of to-day were then covered by water-- “Now land, now lake, and shores with forest crowned.”