
280 STRUCTURAL AND FIELD GEOLOGY _ 

from the 6-inch field-maps, the smaller map is sufficiently 
accurate and detailed for general purposes. | 
The maps issued by national geological surveys are 
seldom on a larger scale than 1 inch to a mile, and are 
usually much smaller. Such maps do little more than 
represent the broader geological features, the distribution 
of the several systems and their larger subdivisions, together 
with the more important developments of igneous rocks, 
leading lines of dislocation, position of ore-deposits, etc. 
They are accompanied, however, by more or less elaborate 
monographs, which contain such detailed information as 
could not be expressed upon the maps themselves. And 
the geology of the regions represented on the latter is still 
further explained by means of horizontal (or profile) and 
vertical sections, the former being constructed so as to indicate 
or represent the shape of the surface and the geological 
structure of the ground, while the latter are designed to show 
in as great detail as possible the succession of important 
geroups or series of strata, such as coal- or ironstone-bearing 
formations. (The method of constructing geological sections 
is set forth in Chapter X XI.) 
Small generalised geological maps on a scale of 10 miles 
to an inch or less, are designed to show merely the dis- 
tribution of the chief rock-divisions, and have usually been 
reduced from larger maps. Sometimes, however, outline- or 
sketch-maps of this kind are original productions, accompany- 
ing the descriptions of hitherto unknown or imperfectly known 
regions. They are meant to do no more than illustrate the 
pioneer work of geological explorers, and do not therefore 
make any pretension to minute accuracy. 
To the student who would become an expert field- 
geologist, topographical maps on a scale of less than 1 inch 
to a mile are of little use. Even a 1-inch map cannot be 
recommended to one who has all his experience to gain. 
The beginner who has the good fortune to commence work 
in this country cannot do better than follow the example of 
our Geological Survey and use the 6-inch Ordnance Map. 
Although this map is large enough to allow here and there 
of notes being inserted, the observer will soon find it necessary 
to use abbreviations, signs, and symbols, For example, 
