



GEOLOGICAL MAPS AND SECTIONS 327 
district. The section in question is a mere diagram, and is 
therefore not drawn to scale, but it exhibits the leading 
surface features and their relation to the underground struc- 
ture. 
There is no difficulty in plotting a profile section on a true 
scale. Ifthe student has laid down his geological lines upon 
the 6-inch map of the Ordnance Survey, all he has to do is 
first to draw a line across the map in the direction to be 
followed by his section. Next, on a separate sheet of paper, 
he draws a line to represent the sea-level. Upon this datum- 
line he erects verticals for the heights of the land traversed 
by his section, which he obtains, of course, from the contours 
on the map. When the extremities of these lines are con- 
nected they give the average form of the ground. If it be 
desirable to reproduce the surface features in greater detail, 
the observer may walk over the ground with his section in 
his hand, and so modify the line as to show the subordinate 
irregularities that appear between the measured contours. 
Usually, however, this refinement is not necessary, when 
the contour lines upon the map succeed each other at 
intervals of 100 feet or less. At the higher elevations of the 
land the intervals between the contour-lines increase to as 
much as 250 ft., and when such is the case the geologist will 
probably consider it advisable to revisit the ground, in order 
to make the upper line of his section represent, as closely as 
may be, the varying configuration of the surface. 
Having satisfied himself as to the correctness of this 
upper line, he then proceeds to insert the dips of the strata, 
and every detail shown upon his map along the line of section. 
Probably the section will now and again traverse places 
where outcrops are not seen, but, the structure of the ground 
having been carefully worked out, he will usually have no 
difficulty in filling in these blanks from the evidence supplied 
by rock-exposures seen elsewhere on the same geological 
horizon. After all the data referred to have been inserted, 
the question arises—how far are the dips exposed at the 
surface to be continued downwards? The depth to which 
we may carry our lines will naturally depend upon the 
geological structure. If our section should traverse normal 
anticlines and synclines, we need have no doubt as to the 
