The Work of the Geological Survey. 
157 
The vertical sections are drawn usually on the scale of 40 feet 
to 1 inch, and are prepared almost entirely to illustrate the succession 
of strata in the coal-fields. Each sheet generally contains more than 
one section. The materials for the plotting of these sections is 
sometimes obtained by actual measurements taken by the surveyor 
himself, but more commonly they are supplied by the lessees or 
managers of the collieries. Sometimes tables of comparative sections 
are given in illustration of the variations in character and thickness 
between the seams of coal, ironstone, or limestone in different parts 
of the same mineral field. 
Occasionally, where a group of strata, though of little industrial 
importance, possesses great geological interest, a vertical section of 
it has been constructed and published in the same style as the coal- 
field section. In this way we have issued some useful sections of 
the Jurassic rocks in Eastern Yorkshire, of the Lower Lias and 
Rhjetic rocks in the West of England, of the Tertiary strata in the 
Isle of Wight, and of the Purbeck group in Dorset. 
Altogether 87 sheets of Vertical Sections have been published 
for the three kingdoms. The price of each sheet is 3s. 6d. 
The Horizontal Sections have always been an important feature 
in the work of the Geological Survey. De la Beche, recognising 
the practical disadvantages arising from the construction of sections 
without any regard to the proportion between height and distance, 
instituted the practice of drawing them on a true scale. He 
adopted the scale of 6 inches to a mile, and invented a system of 
patterns for the different kinds of rock, which, as he was himself 
an artist, are appropriate and effective, for they represent in no 
small measure the general structure of the rocks. The institution 
of such sections in lieu of the distorted diagrams too generally 
employed was of great service to the Survey itself, and also to the 
progress of geology, for it served to correct the evil influences of 
distorted drawing, with regard not only to geological structure but 
to the true forms of the ground. 
When a line of section was chosen and drawn on the 1-inch 
map, it had to be measured on the ground with chain and theodolite. 
This was the invariable practice until the 6-inch contoured Ordnance 
Survey maps came into use. With these maps as a basis, the 
laborious process of chaining the sections is no longer required. 
The section- lines are drawn on these maps and the sections are 
plotted from them. The contour-lines and bench-marks allow the 
line of the surface to be traced with a close approximation to accu- 
racy. But in order to ensure final correctness of detail the ground 
is gone over with the section in hand, and each little feature is then 
put in. 
The sections start from Ordnance datum (mean sea level), but 
where the ground is low and there is consequently not room to ex- 
press what is known of the geological structure above that datum, 
the lines are prolonged below it. The same practice is also followed 
in mining districts. An effort is made to illustrate every great 
