
GEOLOGICAL SURVEYING 293 
112), we have a series of beds dipping from A to B at an 
angle of 45°. The section is on the scale of 6 inches to 
the mile, so that the width of the outcrop between A and 
B is 880 yards, or half a mile. All that we need to do, then, 
is by means of a protractor to draw lines in order to show 
the exact inclination of the beds at A and B respectively ; 
thereafter, another line drawn at right angles to the dip 
from a to & gives the thickness of the series (640 yards), 
From Ato B the beds dip continuously at the same angle, 
but this is not very often the case; more commonly the 
dip is apt to vary in amount from place to place. When 
this is so, all we can do is to take the average angle of 
inclination, and from that we get approximately the true 
thickness. 
The following rule, given by Charles Maclaren in his 
well-known Geology of fife and the Lothians, may be found 
serviceable in estimating thicknesses in the field. If the 
breadth of inclined strata be measured across their outcrop 
at right angles to the strike, their true thickness will be 
equal to ;4,th of their apparent thickness for every 5° of 
inclination. Or the rule may be put thus: divide 60 by 
the dip, and you obtain the fraction which expresses the 
thickness. Thus, suppose a series of strata measures across 
the strike 1200 feet—if the dip of the beds be 5°, their 
thickness is ~;th, or 100 feet ; if the dip be 10°, the thickness 
is 1th, or 200 feet; with a dip of 15° we get a thickness of 
ith, or 300 feet; and a thickness of 4rd or 400 feet, when the 
dip is 20°. The rule is not quite correct when the dip exceeds 
AS 

Thickening and Thinning of Strata.— When the observer 
has completed the drawing of his boundary-lines and out- 
crops, and clearly established the true succession of the 
strata, he will often find that the interval between the out- 
crops of two separate seams or beds varies from point to 
point. In other words, the intermediate strata seem to 
thicken out or to thin away, according as the outcrops are 
followed in one direction or another. Now this apparent 
increase or decrease may sometimes be accounted for, as 
we have seen, by inequalities of the surface, or by variations 
in the angle of the dip. If we have satisfied ourselves, 
