NOTES ON A MAP INCLUDING GREATER PART OF S.E. ENGLAND. 113
over non-drift maps as regards Essex, and the comparative worth-
lessness of non-drift maps of that county. But the maps of which I
then spoke were the ordinary maps on the scale of one inch to the
mile, each of which shows but a small portion of a single county, not
one of four miles to the inch which includes an area in which Essex
occupies about one-fifth. Indispensable as are drift maps to
residents in Essex, who as landowners, farmers, or members of
the County, or of some District, Council are mainly interested in the
uppermost 10 ft., 20 ft. or 50 ft. of strata, the case is altered when
the information required is the general geological structure of a
very much larger district. To the Essex resident, for example,
it may be of little interest to be well informed as to the general
structure of the London Basin, and of the utmost importance
to know whether a certain group of cottages stands upon Boulder
Clay—shown only on a Drift Map—or upon London Clay. If they
are upon Boulder Clay a good water supply is probably obtain-
able for them on sinking some 10 ft. or 20 ft.; if upon London
Clay, sinking to a depth of 200. ft or 360 ft., may, perhaps, be
useless. Again, a remarkable case occurred recently showing the
delusive nature of non-drift maps as evidence on the nature of the
soil of Essex. When the Geologists' Association visited Chelmsford
in June last year, a geological friend, Mr. H. W. Monckton, who was
unable to be present, kindly sent me some notes on the geological
nature of the land which had gone out of cultivation in Essex, and
which he found to be mainly where the soil is London (see ante, p.
70) Clay. He also pointed out that in Mr. Hunter Pringle's Report
on the Ongar, Chelmsford, Maldon, and Braintree districts (which had
been published as a Blue Book, and was largely referred to in the
House of Commons during the debate on Agricultural Depression,
July 11th, 1894), there was the extraordinary statement on p. 36:
"The soil of Essex, with the exception of two small patches, is
geologically classed as London Clay."
Now, this is evidently a mistake which arose from the consultation
of a non-drift map, which would show—like Index Sheet No. 12—
London Clay extending over the whole of Essex except in the north-
west around Saffron Walden and in the south between Purfleet and
East Tilbury. For a single glance at a drift map would have shown
that the Glacial Drift alone covers more of the surface of Essex than
the London Clay does. But, as I have already remarked, the very
fact that the surface of Essex is more largely covered by superficial
H