HOW TO MAKE A GEOLOGICAL SECTION. 
393 
a coal field are made, and are found to assist greatly in plan- 
ning and executing works underground. Accurate sections 
connecting different shafts and different parts of the works are 
necessary, and these, among other facts, prove the nature and 
position of the rocks. But they are not strictly geological 
sections. Sections made in working metal mines are also 
valuable to the geologist, but they are too technical to be 
included among those here described. Occasionally, however, 
sections are obtained illustrating the effect of upheavals and 
movements which are both curious and important in a geo- 
logical sense. An example of this is given in fig. 8, where a 
view of the intersection of mineral veins by dykes and elvans 
introduces and solves a problem of apparent difficulty in 
the mechanics of geology. In this case a a' represents a 
lode or mineral vein, which has been disturbed and removed 
in position by the intrusion of dikes h, x, y, and the move- 
ments that have been connected with their presence. In 
the case represented a a' is a metaliferous vein, and has 
originally been in the position now shown by the upper part 
of a. Several elvans or dykes introduced one after another 
have then disturbed it, and given the whole section the intricate 
appearance it now presents. 
A geological section affords a simple and easy means of 
illustrating the way in which a supply of water is obtained 
from the interior of the earth. Fig. 9 represents several of 
the more common methods, and is a useful diagram without 
strictly copying nature in any one district. In this diagram 
the water-bearing rock of a group of strata, marked 4, is 
represented as cropping out at some points and going down 
to a considerable depth at others. At the point marked 6, it 
comes to the surface on the slope of a hill, and at this point 
the water descending the hill will tend to escape. It will 
then appear as a spring or a row of springs bursting out at a,, 
certain level on a hill side. At another point, marked c, there 
is a fault, and there the water that does not escape at h, as well 
as some of the drainage between h and c, will rise and tend tos. 
get away. At the same point c there will be from time to time 
an overflow from the same water-bearing bed, from the other 
side of the fault. At a third point, d, a crack in the strata, or a 
well sunk to a certain depth, will enable the water retained 
under pressure, above the strata marked 4, to rise either to 
the surface or to a certain height, and thence either flow awa^’ 
or re-enter the strata at a higher level. 
There are many other ways in which geological illustrations 
and facts are conveniently expressed by sections, and for the 
most part they are simple and easily understood. Natural 
sections, or sections obtained on the sides of cuttings, must not, 
VOL. VII. — NO. XXIX. E E 
