598 
WALTORE 
[ Oct, 22, 1885 
D’Omalius down to those of Dumont and his contempo- 
raries, the structure of this country has engaged the 
attention of many able observers, and in its broad features 
is now well known. The map of Dumont, on the scale 
of 1-160,000, is one of the most excellent geological repre- 
sentations of any part of the European continent. But a 
good many years have passed away since its publication, 
and though it remains essentially accurate, it is now 
capable of improvement as regards details. Accordingly, 
after many discussions of the subject, a Commission was 
appointed to undertake a more detailed and exhaustive 
geological investigation of the country. This Commission 
consists of five members of European reputation, viz., 
M. Brialmont, Inspector-General of Engineers, one of 
the most distinguished engineer officers in Europe; M. 
Maus, Honorary Director-General of Bridges, Roads, and 
Mines, who made the preliminary plans for the piercing 
of the Mont Cenis Tunnel; M. Stas, the well-known 
chemist ; M. Liagre, Perpetual Secretary of the Royal 
Academy of Belgium, who measured the geodetic base- 
line of Belgium ; and M. Houzeau, Director of the Royal 
Observatory, whose writings on geological geography are 
widely appreciated. These able and thoroughly repre- 
sentative men of science were constituted as a Board of 
‘Control by which the operations of the Survey were to be 
governed, the practical carrying out of the work being 
placed in the hands of M. Dupont, Director of the Royal 
Museum of Brussels—a geologist of established reputation. 
The work was begun in 1878 with the topographical 
map of the Engineer Department on the scale of 1-20,o0oth, 
or, roughly, about 3 inches to the British statute mile. 
It was estimated that the survey of the whole of Belgium 
on this scale would be completed in seventeen years from 
that date. This detailed map is divided into 430, or, ex- 
cluding the frontier sheets, 369 sheets. Each of these is 
oblong in form, comprising an area of 10 X 8 kilometres, 
or 8000 hectares, or nearly 20,000 English acres. To 
produce upon this larger scale a map which should be 
only an enlargement and rectification of that of Dumont 
was very far from the object of the Commission. It was 
determined to adopt a monographic method of surveying. 
Each important geological system or group of formations 
has been entrusted to one or more specialists, who have 
given particular attention to its investigation, and who 
have been charged with the duty of tracing the same 
system or group completely across the country. Each 
geologist is furnished with two assistants who detach 
rock-specimens, collect fossils, make borings, and in other 
ways save the time and labour of the officer under whom 
they serve. Every actual outcrop of rock is marked on 
the map, and where the rock is fossiliferous the fossils are 
noted and the various paleontological subdivisions of the 
strata are traced, the collector being afterwards sent 
back where more ample collections are thought necessary. 
It was from the first determined that the detailed 
geological map should be not merely a scientific under- 
taking, but a work of as much practical utility as possible. 
Special attention was accordingly given to the soils and 
subsoils, and care was taken to express upon the map the 
variations in the agricultural character of the ground. 
For greater exactness in this respect a system of boring 
was adopted. A stout auger was constructed which 
could be thrust a yard or so into the ground and bring up 
samples of the soiland subsoil. This instrument is made 
use of at intervals of 100 metres along the lines of tra- 
verse, so that the variations in the superficial layers can 
be accurately noted. 
To secure harmony in the work, each officer entrusted 
with the survey of a particular series of strata from time 
to time confers with his colleagues who are engaged on 
contiguous bands, and thus the general geological struc- 
ture of the country is worked out on a uniform plan. 
Up to the present time thirteen sheets have been 
printed off, and many more are in various stages of 
engraving and preparation. It is believed that one-third 
of the entire work of the survey has been completed. 
The ordinary topographical maps of the Etat-major are 
printed from zinc plates, and with their crowded contour- 
lines and rather blurred printing are but ill adapted for 
the insertion of further geological details and the recep- 
tion of colour. The Commission of the Geological Map 
accordingly decided to engrave this map on copper, 
adding new roads and other features, but leaving out all 
non-essential topographical details. By this means an 
admirably clear base has been secured for the delineation 
of the geological structure, while at the same time copper- 
plate engraving has been introduced as a new industry 
into Belgium. Comparing the ordinary sheets with their 
geological equivalents we are struck with the great beauty 
and clearness of the latter. Even for every day topo- 
graphical use they are immeasurably superior. 
One of the great problems of geological cartography is 
how best to pourtray at once the superficial accumula- 
tions and the solid rocks that lie underneath these. In 
this country it has been found practicable on the detailed 
six-inch maps of the Geological Survey to represent the 
surface-deposits by various kinds of stippling on the 
copper plates, the alluvia and the solid rocks being ex- 
pressed by tints of colour. On the one-inch maps, how- 
ever, which show the surface features by shading, this 
method cannot be employed. It has accordingly been 
necessary to issue two versions of each sheet of the one- 
inch map—one showing the solid rocks, the other repre- 
senting the distribution of the various drifts and other 
detrital accumulations. These maps are coloured by hand, 
and are often of great beauty, but of course are somewhat 
expensive, more especially as two editions are needed 
to complete the representation of each district. M. 
Dupont deserves the admiration of geologists for having 
solved this difficult problem in an altogether novel way, 
and for having produced a series of maps which 
will probably inaugurate a new departure in geo- 
logical cartography. His principle is to represent all 
the geological formations of a district, ancient as well 
as modern, upon the same sheet. As the superficial 
accumulations extend across much the largest area of 
ground, they are shown by various broad washes of colour 
over the tracts which they respectively cover. These 
colours, though they necessarily spread over most of each 
sheet, are kept so subdued in tone that they do not 
interfere with the easy legibility of the stronger tints 
employed to denote the underlying solid rocks, Every 
actual outcrop of these rocks is marked by a patch of the 
colour chosen for the particular formation. We thus 
note at a glance the localities where the rocks of that 
formation can be seen at the surface. At these outcrops, 
signs are inserted to mark the dip, and any lithological 
or paleontological subdivisions which have been noticed, 
and regarding which a detailed legend on the sides and 
bottom of the map gives ample explanation. So far the 
map is‘merely a transcript of what is observed in nature. 
But it is of course necessary to express the limits of the 
several rock-groups. And it is here that M. Dumont’s 
ingenuity is most remarkable. He shows these limits by 
dotted lines, the dots varying in strength according to 
the importance of the limit which they define, and by 
strips of colour. Each stage has its margin defined by a 
shaded strip of its characteristic colour where the actual 
boundary is concealed, while where the junction of two 
stages or sub-stages is actually seen on the ground, the 
colours are not shaded, but of the full strength. The eye 
can thus easily follow the windings of such sub-division 
across the map, and can at a glance mark where the 
actual exposures are to be observed on the ground. 
As the maps are chromo-lithographed it is quite simple 
to secure harmony of tone and great clearness and 
accuracy. We at once perceive what is actual observa- 
tion and what is inference. One is put in possession of 
