THE FIELD NATURALIST'S KNOWLEDGE. 63
Sandhurst and Woking districts where almost the only trees to be seen
on the Bagshot Sands are conifers, and so too on the Tunbridge
Wells Sands of Eridge extensive pine woods occupy the land, while
they are absent in the adjacent valley where the Wadhurst Clay
gives land supporting magnificent timber trees of various kinds.
Although the rhododendron is a Boreal shrub and flourishes in
sub-arctic regions, yet in England it nowhere grows more luxuriantly
than in the south, where on the heathy covering of the Bagshot
Sands of Knap Hill it seems to find its most fitting soil.
The suitability of certain soils to certain produce is of course
familiar to every farmer, but the connection between the surface soils
and the underlying rocks is not so generally known, and yet it is very
distinct, and in some districts very conspicuously indicated by the
vegetation as in the examples that have here been cited, as well as in
many others. A long strip of country in the south of the county of
Sussex affords another excellent illustration of this. Extending east
and west are narrow outcrops of Lower Greensand, Gault and Upper
Greensand, with a line of Chalk hills, the South Downs, rising from
the vale to an elevation of 600 feet above the level of the sea, and so
giving a fourth formation having an east and west surface extension.
Thus there are four parallel bands of land formed of four separate
formations. On the Lower Greensand there are extensive furze-
covered commons and heaths with numerous woods and much culti-
vated land. Along the south side of the Lower Greensand area
extends the Gault Clay outcrop, forming a narrow shallow valley of
heavy land covered with grass of a darker tint than that on the adja-
cent land, and studded with oaks almost exclusively. As the ground
begins to rise again the Upper Greensand at the base of the Chalk
gives with the wash of the calcareo-argillaceous lowermost Chalk beds
fine arable land, while the Chalk forming the higher land along the
sides and on the summit of the Downs affords the well-known South
Down ground covered with the fine grass on which the South Down
sheep feed and depend. It ought therefore to be felt by botanists
that it is not enough to record the soil and position of a plant, but
that the underlying geological formation ought to be stated also.
On the coast of Italy are two well-known areas near together,
having a similar climate, and being similarly conditioned generally
except in their geological structure, which is markedly different.
These areas are, one, the cultivated area on the slopes of Vesuvius, the
other, the Island of Capri. The first is entirely volcanic, the second