216 
ST. ALBANS AND ITS NEIGHBOURHOOD. 
worked for brick-making. Where, as usual, it lies upon clay- 
with-flints, it appears to be a reconstructed Tertiary deposit. 
It is widely distributed over our district, but is nowhere of any 
considerable thickness except at the Bennett’s End brick-fields 
near Lever stock Grreen. Here it is seen to lie on a very irregular 
surface of the Chalk coated with clay-with-flints, and to have in 
its upper portion masses of conglomerate and irregular layers of 
flint- and quartz-pebbles. 
The rivers have as usual deposited gravel and alluvium in 
their beds, but there is nowhere a great spread of the latter as 
there is lower down their valleys. There is very little peat in 
the district, it only being known to occur in marshy places at 
No-Man’s Land, Colney Heath, and Bricket Wood Common. 
From the character of the gravel at No-Man’s Land and the 
presence of peat, the gravel there appears much more likely to 
be of fluviatile than of glacial origin, having most probably 
been deposited by the stream which formerly flowed through 
Harpenden and into the Colne at Colney Heath. 
Further information on the geology of this district may be 
found in our ‘ Transactions,’ especially in the reports of our 
Field Meetings, every locality of geological interest in this part 
of Hertfordshire having been visited and its geological features 
described. 
IY. HYDROLOGY. 
Those who are unacquainted with the physical features of 
a Chalk district must view with surprise the long stretches 
of the rivers of Western Hertfordshire without a tributary 
stream, and their numerous dry subsidiary valleys. That these, 
as well as our river-valleys, have been formed by the action of 
water does not admit of doubt, and we are led to infer that 
water must at one time have flowed down them. Again, our 
Chalk rivers do not now rise at the head of their valleys, 
showing that a shrinkage has occurred in their length which 
must have occasioned a shrinkage in their volume. 
The accompanying table gives the results for each summer 
and winter of experiments on percolation at Rothamsted for 
the 39 years April, 1871, to March, 1910, with gauges one- 
thousandth of an acre in area. Of the three percolation gauges, 
of 20, 40, and 60 inches in depth, the results of the 60-inch 
gauge are alone given. As the surface of the soil in the gauge 
is kept free from weeds by frequent hoeing, these experiments 
give a much greater percolation than occurs under natural 
conditions, but they are of interest as showing the variation 
which has taken place from year to year, and the great difference 
maintained between the summer and the winter percolation, 
even under circumstances which must make this difference much 
less than it is in a natural state of the soil. 
Almost the whole of the Chalk formation, and especially the 
Upper Chalk, is very permeable, the rain which falls upon it, 
