(3.) At a depth of 7 feet the first horizontal beams of the gangway were encountered. (4.) At 10 feet deep the base of the gangway was supposed to be reached. The canoe found at Lochlee, though in a different part of the lake-bed, was also buried to a depth of from 5 to 6 feet. Regarding the marked change in the upper portion of the sedimentary deposits in both these lakes, perhaps no better explanation can be offered than that already suggested at page 151, viz., that it is due to the disappearance of the primeval forests which formerly covered their drainage areas, and the conversion of the land to agricultural purposes, when part of the broken-up soil and clay would be washed down into the lake-basin, and so become mixed with ordinary silt. Another possible explanation may be found in the gradual filling up of the lakes, and their entire conversion into marshes, covered with rank grasses and other aquatic plants whose decomposition would henceforth take the place of their former mossy sediment. The proximity of Buston and Lochlee, as well as the topographical similarity in the surrounding landscapes, would indicate that this great change took place about the same time in both these localities. Long before this, however, both canoes were finally sunk or abandoned, the interval of time being measured by no less than 4 feet of a deposit. Again the entire time that has elapsed since the gangway at Lochlee was laid down till the lake was drained, and tally ceased to be kept, is represented by 10 feet. _Subsidence of the Crannogs._--Before quitting this somewhat speculative line of research, one other subject remains to be discussed, viz., the submergence of the Lake-Dwellings. This phenomenon is a uniform feature in all those hitherto examined, and though the causes of it have not been much inquired into by previous writers, they will, I think, be found of sufficient importance to merit the attention of both the antiquary and the geologist. There are just two immediate causes to which this result can be assigned: viz., either a subsidence of the surface of the island, or a rising of the waters of the surrounding lake; or, as may happen sometimes, a combination of both causes. The physical agencies that are likely to operate in producing a subsidence of the island may be categorically stated thus:-- (1.) A compression of the island due to consolidation or decay of its structural materials. (2.) A sinking of the whole mass of these materials in soft mud as a direct result of weight. (3.) A general compressing and sinking of the sedimentary strata of the lake-bed.