This has been done very carefully by the officers of the Geological Survey and other eminent authorities in England and Scotland, and the result clearly shows that since the last glaciation left the country buried in a thick mantle of boulder-clay and drift, such an amount of denudation, and such movements of elevation and depression have taken place, as must have required a great lapse of time. The most complete attempt at an estimate of this time is that made by Mr. Mellard Read of the Geological Survey, from the changes proved to have occurred in the Mersey valley. In this case it is shown that the valley, almost in its present dimensions, must have been first carved out of an uniform plain of glacial drift and upper boulder-clay by sub-aërial denudation; then that a depression let the sea into the valley and accumulated a series of estuarine clays and silts; then an elevation raised the whole into a plain on which grew an extensive forest of oak rooted in the clays; this again must have subsided and let in the sea for a second time, which must have remained long enough to leave a large estuarine deposit, and finally the whole must have been raised to the present level before historical times. The phenomenon of the submerged forest is a very general one, being traced along almost all the sea-coasts of Western Europe, where shelving shores and sheltered bays favour the preservation of patches of this primæval forest. It testifies to a considerable amount of elevation and subsequent depression, for its remains can be traced below low-water mark, and are occasionally dredged up far out to sea, and stately oaks could not have flourished unless more or less continental conditions had prevailed.