vegetation belonging to the latter deposit and the pelophilous or clay-loving species characteristic of the former.
On Stoke Common we therefore find such plants as the all-seed (Millegrana Radiola), the dwarf willow (Salix repens), the alder buck- thorn (Rhamnus Frangula), the meadow thistle (Cnicus pratensis), the heaths Erica Tetralix and E. cinerea, the ling (Calluna Erica), the petty whin (Genista angtica), the dwarf gorse (U/ex minor), the upright pearl wort (Cerastium quaternellum-Mcenchia), the sedges Carex binervis, C. echinata, the grasses Nardus stricta, Aira prcecox, A. caryopbyllea, Deschampsia flexuosa, and Festuca ovina var. paludosa.
On the elevated outlier at Lane End there is some marshy ground where the marsh helleborine (Epipactis palustris), the bog pimpernel (Anagallis tenella), the marsh lousewort (Pedicularis palustris), the sedges Carex pulicaris, C. Jiava, C. ecbinata, C. Goodenowii, and C. panicea, grow in very near neighbourhood to the petty whin, the dwarf gorse, the ling and the heath form of Orchis maculata, i.e. var. ericetorum, and where the small winter-green (Pyrola minor) grows in short turf bordering a wood in which the golden saxifrage (Chrysosplenium oppositifolium) occurs.
The Drift. A casual glance at the geologic map, as coloured to show the limitations of the various strata which come to the surface in the county, does not reveal, as has already been stated, the true surface soil over a great part of the area. In the north we find that both the Oolite and the Oxford and other clays are sometimes covered with or obscured by masses of clay full of pebbles, or by more or less extensive patches of sands or gravels, and this is specially the case in the portion drained by the Ouse ; indeed that river gives the name of the ' Ouse gravels' to them. The influence of this covering upon the vegetation has already been referred to. The brick-earth and clay with flints, differing as it does materially from the bed rock in chemical composition as well as in physical characteristics, also has great influence in changing the character of the flora as we have previously seen.
High-level and Low-level Alluvium When rivers flow with a gentle fall across flat country they are usually margined by tracts of flat meadow-land, which are composed of materials carried down by the stream and dropped whenever a slackening of the current prevents the matter being carried further. Such deposits are known as Alluvium. They may be gravelly, loamy or clayey. Their component parts are purely local, being derived from the immediate neighbourhood of the stream, so that in the meadows of Marlow or Windsor the alluvial gravels contain a large percentage of the cretaceous rocks through which the river has cut its way, and the only foreign elements are such as are derived from the Drift or High Level Gravels which may have been cut through and reassorted and mixed with those of purely local origin. Hence we notice a remarkable difference between the vegetation of the alluvial meadows of the Thames and of the Ouse ; the latter being chiefly occupied by mesophytic plants, that is, such as are almost ubiquitous or common to various situations, or by
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