306 
Agricultural Notes on Hertfordshire. 
Cambridge. The rivers Coin and Lea flowing in opposite 
directions in part of their course, form a sort of natural division 
between these two districts, though they do not strictly deter- 
mine the limits on either side. 
In the southern district, the London Clay, is mostly marked 
by low, rounded undulations, broken by tortuous watercourses, 
which provide the natural escape for surface-water, and a ready 
outfall for artificial drainage. 
Swallow-holes. 
The upper levels are very frequently covered with beds of 
gravel, which retain a certain quantity of water for the supply 
of shallow wells, which, as on Bushey Heath, attract a con- 
siderable population. The water also finds vent in land-springs 
at the junction of the gravel with the clay, the feeders of the 
brooks which run into the rivers Coin and Lea. Occasionally 
these waters pass, in their course towards the valley, the outcrop 
of the sand which underlies the clay beds of the Plastic clay 
formation; it then sinks by swallow or " swilly holes" into 
the subjacent chalk, and goes directly to augment the springs 
whence the rivers derive their perennial sources. Very large 
volumes of Avater so sink into the earth, and the mischief which 
would arise from the flooding of these brooks in winter is thus 
much abated. It has been suggested by very high authority 
that the perennial supply of water to rivers might be materially 
augmented if artificial means were used to facilitate the absorp- 
tion of these waters. Very remarkable instances of this natural 
drainage may be seen more or less along the outcrop of the sand 
beds of the Plastic clay formation in the parishes of Bushey and 
Aldenham. In the watercourse which leads from the reservoir 
at Elstree, it has been found necessary to stop these swallow-holes 
to prevent waste. 
The construction of artificial swallow-holes deserves our con- 
sideration as a means both of maintaining a perennial supply of 
water to our rivers, and also of facilitating drainage operations on 
a system suggested more than a century ago by Elkington. 
The soil of the upper levels of the clay district marked by the 
rounded flint-pebbles embedded in the sand is wet and unkindly, 
not capable of bearing grass of any value, and ungrateful under 
the most liberal treatment as arable land. This gravel, with its 
characteristic blue-pebble, is transported in many cases below the 
higher levels, where the sterility of the soil is in proportion to 
the thickness of the bed. Where the London clay comes to the 
surface it forms a stubborn soil, which, however, by draining and 
a liberal treatment is made to grow abundant crops of grass. It 
also favours the luxuriant growth of oak, elm, and ash timber. 
