The Work of the Geological Survey. 
147 
brick-earths, clay-with-flints, and other deposits, that form so 
marked a feature on the Chalk Downs. Erom the Chalk districts 
westward across the Jurassic, Devonian, and older formations, even 
to the farthest headlands of Cornwall, every rock is more or less 
buried under a covering or “ head ” of its own decayed material. 
Sometimes, as on the Oolitic strata of Dorset or the killas of Corn- 
wall, this upper decayed layer may be traced as a yellow or orange 
band, varying from a few inches to many feet in thickness, con- 
forming to the shape of the surface, and presenting a singular con- 
trast to the black horizontal shales of the one coast and the purple 
vertical slates of the other. In the interior, where natural or 
artificial exposures of the rock are sometimes scarce, the spread of 
this mantle of disintegrated material is a serious impediment to the 
mapping of what lies underneath it. 
2. But it is the second or transported series of surface-deposits 
which chiefly engages the attention of the Survey. In mapping it 
an effort has been made to discriminate each of its members, to 
trace out their relations to each other, and to ascertain the con- 
nected geological history of which they are the records. At the 
same time, regard has been had to the practical applications of the 
inquiry, the connexion between soil and subsoils has been kept in 
view, pervious and impervious deposits have been distinguished, 
and an endeavour has been made to collect and embody on the maps 
as much information as possible concerning the practical bearings of 
the surface-geology. 
As an illustration of the detail into which the mapping in this 
department has been carried, I may mention that under the 
single term “ alluvium ” we now discriminate and indicate by 
separate signs and colours a large number of distinct deposits. 
Thus, there is a group of freshwater alluvia, beginning with the 
present flood-plains of the rivers and rising by successive terraces 
to the highest and oldest fluviatile platforms. Deposits of peat are 
separately traced, and tracts of blown sand are likewise mapped. 
Then there is another series, of marine alluvia ranging in position 
and age from the mud of modern estuaries and the sands of flat 
shores exposed at low water, through a succession of storm-beaches 
and raised beaches, up to the highest and most ancient marine 
terraces 100 feet or more above the present level of the sea. Re- 
garding the origin of some of the high-level gravels there is still 
much uncertainty, but the Survey has taken the first necessary step 
for their ultimate explanation by carefully tracing their distribution 
on the ground. 
But the most abundant and complex group of superficial deposits 
is that which may be classed under the old name of glacial drifts. 
These have been mapped by the Survey in detail, and much of the 
progress of glacial geology in this country has been due to the sedu- 
lous investigation thus required. The ice-strise on the solid rocks 
have been observed over so much of the country, that maps may 
now be constructed to show both the march of the main ice-sheets 
