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NEW PUBLICATIONS. 365
mechanical constitution. The ridge of chalk-down that forms the rim
that encloses the tertiary strata of the London basin, stretching from
Hampshire and Wiltshire, through Berkshire and Hertfordshire to C'ara-
bridgeshii'e, and the north-west of Essex, only just touches the extreme
limit of Middlesex at two points. On the south of this, filling up rather
more than the northern half of the county,—reaching down on the east,
within the metropolitan limits, to Eegent's Park and Holloway,—the
Loudon clay fills up the whole of the low levels, capped only with the
barren sandy and gravelly beds of the Middle Eocene in a few places
on the ridges, as at Harrow and Harapstead Heath. The greater part
of this clay tract is covered with soil that is far too tenacious to be fit
for arable cultivation. " In few counties," writes Mr. Clutterbuck
(see p. XXV.), " is the meadow and arable land so nearly divided, or the
extent so clearly defined ; and though not without exceptions, the sur-
face occupied by the London clay and the valley-drifts respectively,
determines the extent under grass and under the plough. The part of
the county in which the London clay is at or near the surface consists
of gently rising hills, with small valleys gradually worn away by the
surface drainage. In the farms, all operations are made subservient
to haymaking for the London market." In the southern tract, which
fills up rather less than half of the county, the surface beds are valley-
drift (gravel, brick earth, and alluvium), and the soil is much more
tractable and fertile. The eastern part of this tract is now nearly all
built over. Passing westward to Chiswick, Hammersmith, Isleworth,
and Brentford, what is not taken up by houses, roads, and parks, is
almost all occupied by market gardens ; and this leaves only on tlie
west a tract of about ten miles across each way between Tvvickenliam,
Staines, and Uxbridge, in which corn is grown to any considerable
extent. Our authors' sketch of the physical geography and climate of
the county is very full and clear. The only point on which we have
any fault to find is, that they have not understood clearly the rela-
tionship of the British to the European flora as influenced by climate.
As this is an important point, and their misconception will very likely
lead others astray, we will quote their paragraph on this subject, and
interpolate a running criticism on the sentences.
" Plants which would not bear complete exposure to frost will often
survive, with slight shelter, frosts of short duration ; and near the
western coasts, where the influence of the sea has greater effect, espe-