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Rh Western railway, and although not scientifically correct we adopt the railway as the separating line from the Thame district hereafter to be described as far as to Cheddington station; thence our dividing line is traced by Wingrave, Aston Abbots, and passing to the east of the Cottesloes to Stewkley and Mursley, where the Ouse district limits it on the western side till it touches the starting-point near Great Linford; for the last few miles, that is from Simpson to Linford, the Grand Junction Canal is adopted as the dividing line, but strictly speaking it is not quite accurate, as a small portion to the west of the canal near Willen actually drains into the Ouzel but it is thought better to choose a definite rather than an obscure line in this instance.

The country comprised within these limits is remarkable from the fact that the streams cut through or across the lines of strata, so that in it are represented the Lower Chalk, the Upper Greensand, the Gault, the Lower Greensand, the Kimeridge and Oxford Clays, as well as the Ouse Drift Gravels. Not only are the geologic strata thus richly represented, but the scenic character offers a very pleasing contrast to the dull monotony of so much of the Ouse district, and the vegetation is also of a most interesting nature. Some parts of the Dunstable Downs are among the highest points of the county, being over 800 feet, while at the juncture of the Ouse and Ouzel the surface of the water is only about 180 feet above sea level. The Chalk downs are in places covered with aboriginal turf, in which grow such plants as the ragwort (Senecio campestris), the orchids Orchis pyramidalis and Ophrys apifera, the squinancy-wort (Asperula cynanchha), the horse-shoe vetch (Hippocrepls comosa), the lady sfingers (Anthyllis Pulneraria), the grasses Bromus erectus, Avena pubescent and A. pratensis, and in one place above 700 feet in altitude the adder's tongue (Ophioglossum vulgatum), and the box (Buxus sempervirens) has been claimed to be a native plant. In some places the downs have been brought into some kind of agrarian cultivation, and in these arable fields we have glaring masses of colouring produced by the white mustard (Brassica alba) and the sainfoin (Onobrychts viciafolius), and as more interesting constituents to the botanist, Fumaria densiflora, F. Vaillantii and F. parviflora, the pignut (Carum Bulbocastanum), which is found in no other district of the county, the candytuft (Iberis amara); while the grasses Festuca rigdia and Phleum nodasum are frequent plants.

On the Upper Greensand two umbelliferous plants, Carum segetum and Caucalis nodosa, are found, especially on dry sunny banks near villages. When we come to the Lower Greensand, which is so well represented between Leighton Buzzard, Heath and Great Brickhill, and still more interestingly between Great Brickhill, Little Brickhill and Bow Brickhill and Woburn Sands, we see the richest botanizing ground in north Bucks. This portion of the county is very picturesque, and in places clothed with pine woods, which reach an elevation of about 520 feet. The flora of the Brickhill woods on the Greensand has already been rather fully described, but we may refer to the flora of some other parts which has not already been mentioned. Between Great Brickhill and Heath there are some very interesting cornfields and heathy ground. In the former the rather rare cudweed (Filago apiculata) is plentiful, and the cress Teesdalia nudicaulis also occurs in abundance; the tower cress (Arabis perfoliata) is very local, as is its wont, and the swine's succory (Arnoseris pusilla) is limited to a very small area; one or two forms of the wild pansy were found here for the first time in Britain, and the chickweed (Cerastium semidecandrum) occurs as the variety viscosum, and also in another variety which is apparently undescribed. In the heathy ground the buck's-horn plantain (Plantago Coronopus), the sheep's scabious (Jasione montana), the golden rod (Solidago Virgaurea), and the pearlwort (Sagina ciliata) are found.

In a marshy wood and in moist, open ground in this neighbourhood, the marsh fern (Lastrea Thelypteris), the golden saxifrage (Chrymplenium oppasitifolium), the sedges Carex paniculata, C. disticha and C. rostrata grow. A pond near Little Brickhill, and almost at the highest level (namely 513 feet), is remarkable for its containing the pondweed (Potamogeton alpinus), the sedge Carex Pseudo-cyperus, the water buttercup (Ranunculus heterophyllus), and the charads Chara hisplda and Tolypella glomerata.

The adjoining pasture fields have the lady's mantle (Alchemilla vulgarii) and quantities of the orchid Orchis morio, which is locally abundant in the district. Duncombe Wood affords the herb Paris (Paris quadrifolia}, the gromwell (Lithospermum officinale), the butterfly orchid (Habenaria chloroleuca). The hedgerows and waysides in the district of the Brickhills have several interesting species including the mint (Mentha longifolia), the calamint (Calamintha officinalis or montana), the hound's-tongue (Cynoglossum officinale), the bur parsley (Anthnscus vulgaris or Cerefolium Anthriscus), and the downy rose (Rosa tomentosa or mollissima, not mollis).