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Rh districts, and in almost every case the delimitation of the districts is made by adopting the different drainage areas of the smaller streams, or by dividing the larger into two or more parts. By this means, it is con- tended, more valuable scientific results are obtained than if an artificial system were chosen, or one based upon the geological divisions, or even one in which the divisions were made to represent the various soils. The objection to the plan based upon the geological strata is that so much of the area is obscured by surface deposits, which, as we have seen, mask the character of the strata underneath ; and although in my opinion apian in which the surface soil itself were used to determine the standard would be more valuable, it is quite true that our knowledge of the subject is not yet sufficiently perfected to make it available. The plan adopted here is therefore one based upon the river drainage, notwithstanding the difficulty that sometimes is felt in the separation of portions of the country where the water-parting is obscure, and the more serious objection which is experienced when, as in the case of the Ouzel or Thame, which run transversely to the geological strata, each subdivision will contain several different strata. Moreover, instead of a more or less uniform scenic effect and a fairly uniform vegetation, which would be the case were a stratum such as the Oxford Clay selected, it is obvious we shall in such an instance as that of the Thame have all the varying effects, which different altitudes and soils give to the districts traversed by that stream in its course from its origin in the Chalk hills of Wendover or the Green- sand of Brill through the clays to the river Thames. On the contrary in choosing this plan we keep in harmony with the arrangement adopted in works on the botany of the bordering counties, and the student of plant distribution will be enabled to investigate more easily the flora of the smaller river basins of the midlands.

Unlike Berkshire, which is wholly contained in the Thames basin, Buckinghamshire has two important river systems, those of the Ouse and the Thames. These therefore form our two great divisions, but the area drained by the Ouse is capable of being further sub-divided into two portions, namely that drained by the Ouse itself and that drained by its tributary the Ouzel. In eliminating the country drained by the latter stream we shall include in the Ouse district proper the country on the Liassic and Oolitic strata. These strata are found also in those portions of the bordering counties which are also in the Ouse drainage. The country drained by the Ouzel is much more varied in its geological character.

The district No. 1 drained by the Ouse has its counterpart in district No. 3 of my Northamptonshire Flora and in district No. 2 of my Oxfordshire Flora. Roughly speaking its configuration is as follows : The Ouse rises at Ousewell near Brackley, and leaves our county near Olney. About a mile from the pleasant town of Brackley the Ouse forms the county boundary of Oxfordshire as far as to Water Stratford, where the line of delimitation from the Cherwell or rather the Ray drainage of Oxfordshire is a line drawn along the Roman road to Newton Purcell, then across country to Goddington, crossing the London and North-Western Railway near Marsh Gibbon station, passing by Calvert station on the Great Central Railway