HOW TO MAKE A GEOLOGICAL SECTION. 
387 
the beds. In other cases they only afford materials from which 
geological sections may be constructed. Simple as they seem, 
the reduction of such observations to a continued section involves 
some knowledge of mathematics as well as of geology, and many 
careful observations made over a great extent of country. Sec- 
tions do not indeed always represent a long distance. It depends 
on the purpose for which they are taken whether they express 
the conditions of stratification for a few 3^ards or hundreds of 
yards, or whether they refer to distances of miles or even hun- 
dreds of miles. 
Geological sections are the results of minute observation on 
the position of strata, and combined with geological maps they 
lay bare the structure of a country. They are in an important 
sense the language of geology, and they are no less valuable in 
practica-l geology, where they determine important questions in 
engineering, than in the more popular and elementary part of 
the science, where they help the student to understand the great 
facts of the superposition and disturbance of strata. 
The first point to be determined in preparing a geological 
section in a given district is to find the dip and strike of the 
strata generally over the whole district. The strike of the 
strata is direction of a horizontal line on ao exposed natural 
face of rock. It is best obtained by the assistance of a spirit 
level. A long straight staff laid on the exposed surface of the 
rock is often convenient to obtain a satisfactory idea of the real 
plane of stratification, and the spirit level being placed on this 
staff may be turned round with it till it shows a level. The 
compass bearing of the direction thus obtained is the strike of 
the beds. The inclination downwards of a line at right angles 
to the strike, measured by a small quadrant attached to the 
spirit level, will give the cli'p of the bed, or the angle it makes 
with the horizon. It is not necessary that observations of this 
kind should be made with minute accuracy, but they must be 
repeated frequently wherever there is an opportunity, the position 
of the place of observation being noted down (on as good a local 
map as can be obtained) by a small arrow pointing in the 
direction of the dip and marked with the number of degrees. 
It must not be assumed that the dip is the same at places some 
little distance asunder merely because the eye detects no change ; 
it is, on the contrary, very seldom that there the dip is absolutely 
constant, and small changes of dip sometimes indicate faults and 
disturbances of serious import. The dip is rarely identical even 
in different parts of the same quarry. 
The diagram (fig. 1 ) in the annexed Plate illustrates the case 
just described. By a few observations in the field, it is found 
that the general strike of the strata is nearly N\Y. and SE., and 
that in the southern part of the tract represented (near the 
