CHAPTER IT. 
METHOD OF mVESTIGATIOTs T . 
FIELD WORK. 
Geologic field work is not practicable in some portions of the Appa¬ 
lachian region unless the area to be investigated is generally dissected 
by erosion to a depth of 100 feet or more, bringing the outcrops of 
different key strata to the surface. 
Geologic work of this nature generally follows the making of care¬ 
ful topographic surveys, such as are now being carried on by the 
Geological Survey. In the topographic work, bench marks based 
on precise level lines brought from the sea are established. This is 
done by means of primary level lines run in circuits, with a closure 
restriction permitting an error of only 0.05 foot in the square root 
of the linear distance in miles. These primary lines are run in such 
a manner as to establish bench marks at intervals of not more than 6 
miles from one another, leaving no point within the area distant more 
than 3 miles from a permanent bench. 
In carrying on the geologic work proper, level lines based on the 
permanent benches were carried over each road and in many cases 
up stream channels and along the crests of ridges, with an accuracy 
of instrumental work that insures the closing of circuits with errors 
of less than a foot. In the course of this work the elevation was 
taken of outcrops of all beds that in any way conld be recognized as 
marking geologic horizons. As the elevation of each outcrop was 
determined, its horizontal position was located on a topographic 
map carried by the levelman. 
This combination of geologic and topographic work results in a 
mass of extremely accurate data for determining the intervals 
between important beds of rock and also the geologic structure or 
“ lay ” of the rocks. 
«v 
For obtaining the vertical distance between well-recognized beds, 
certain outcrops were selected whose elevations could be compared 
directly one with another. Only such outcrops as are horizontally 
near together were selected for comparison, except in cases where the 
outcrop of one bed could be compared with two or more outcrops of 
another bed showing on different sides of the first exposure. The 
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