

GEOLOGICAL SURVEYING 279 
and exactly are geological positions determined. To be of 
any practical use, a good geological map ought to exhibit the 
following features, viz. :— 
(a) The superficial areas occupied by geological systems 
and their chief subdivisions—the mutual boundaries of the 
several groups or series being accurately delineated. 
(4) Individual seams, beds, or formations of economic or 
scientific interest and importance, such as coals, limestones, 
ironstones, etc.; the position of available building-stones, 
etc.; the best sources of underground water-supply; the 
general character and distribution of superficial accumulations, 
subsoils, and soils. 
(c) Igneous rocks—effusive being clearly distinguished 
from intrusive rocks. 
(2) Faults, and all fissures which may be supposed likely 
to contain ore-formations. 
(e) Dips should be everywhere carefully inserted, so as to 
show exactly the direction and degree of inclination of the 
Strata. 
A map containing these data would enable a geologist, 
who might never have visited the region represented, to 
understand at a glance the geological structure. From the 
details given, he could measure the thickness of the strata, 
and ascertain the depths from the surface at which particular 
seams or beds might be expected to occur at given points. 
He would be in a position to indicate where an underground 
water-supply might be tapped by borings—all this, and much 
more, a carefully constructed geological map will reveal to 
anyone who has the skill to read it. Only large scale maps, 
such as those issued by the Geological Survey of Great 
Britain, are sufficiently detailed to be used in this way. The 
field-observations of the Survey are plotted on the larger 
Ordnance Map (6 inches to a mile), and the sheets repre- 
senting the more important parts of the country are published 
on that scale. The several geological systems and their sub- 
divisions, and the general structure of a region, however, can 
be quite well represented on a smaller scale. The Geological 
Survey, for example, issues a general map, on the scale of 
I inch to a mile, the information given on which is, of course, 
taken from the larger map. Having been carefully reduced 

