WITH ESPECIAL REFERENCE TO THOSE OF ESSEX. 31
Clay, marking the outcrop of the lower Tertiary Beds. On the
Drift Map of 47, the brown of the London Clay has become so
obscured by the light blue marking the presence of the Boulder Clay
above it as to be scarcely perceptible anywhere but in the river
valleys. And though more London Clay is visible in Sheet 1, it is
only here and there that any considerable unbroken area of it is seen,
the largest being the Epping Forest district, that north of the Langdon
Hills, and the tract due south of Maldon. And instead of London
Clay extending to the alluvium of the marshes, we have a broad belt
of country west of the Lea and north of the Thames covered by
old river gravel, with occasional patches of brickearth. In addition
the drift map discloses the existence of sundry patches of gravel
of various ages capping the hills here and there, which do not appear
on the driftless edition. It may seem somewhat singular, that while
the superficial beds generally are ignored on the non-drift maps, an
exception has always been made as regards the alluvium of the
marshes. The explanation is that the Geological Surveyor neces-
sarily walks carefully along all rivers and brooks, and that the limits
of the marshland are mapped without any difficulty whatever while
he is doing so. On the other hand, it must be apparent to all who
compare the drift and driftless editions of Sheets 1 and 47, that the
former must have involved at least three or four times as much labour
as the latter.
It is much to be wished that the maps and other publications of
the Geological Survey were better known and more easily obtainable
than they now are: that the authorities of the Science and Art
Department would take some steps not only to advertise their exis-
tence, but to make it more easy to see and procure them. Of course
persons living in or near London can easily obtain such as they may
require at Stanford's, Charing Cross, or get any information about
them at the Museum of Practical Geology, Jermyn Street. But
people living fifty miles or more from London cannot always spare
the time and money involved in a visit to the places just named
whenever they want either maps or geological information, and without
some inspection of the maps and memoirs themselves it must often
be difficult or even impossible to know what order to give a local
stationer. Perhaps some day examples of the maps, sections, etc., of
the Geological Survey dealing with any given county or district may
be visible, or even purchasable, at the Post-office of every considerable
town in that district. For it is obvious that the possession of