HOW TO MAKE A GEOLOGICAL SECTION. 
389 
comparing two such sections, taken at different pits in the same 
district, the position of the strata at considerable depth is made 
out. Such sections are equivalent to observations made in 
very deep cuttings, and extend to many hundred or even thou- 
sand feet. They represent hundreds of strata. Where the strata 
are tolerably regular, and where the general nature of the dis- 
turbances is already ascertained, a great deal of most valuable 
information may be obtained from these measurements and 
statements. They tell us on a tolerably large scale the average 
dip and the average thickness of the beds ; and by comparing two 
such sections, and connecting them on paper, exceedingly useful 
practical knowledge is obtained, and further investigations 
suggested. 
Horizontal and vertical sections strictly correspond with each 
other, and are only so named because the observations for 
making the former are taken at the surface, and those for the 
latter from actual sinkings. By the former we obtain a series of 
facts concerning strata drawn from surface appearances, but often 
involving a thickness of many thousand feet. This is only 
obtained, however, when the strata have been tilted, so that, 
within a moderate horizontal distance, a number of beds under- 
lying each other crop out in succession. This will be seen by 
reference to fig. 4, where, if the length of the section (a b) 
represents a mile, the height above the base line (ad) will be 
about 300 feet, and the length of the line ac, which represents 
the total thickness of all the strata cropping out between a and 6, 
will be about 2,500 feet. Thus a shaft (a e) sunk from a to the 
depth of 2,500 feet will give no more information concerning the 
strata, than would be obtained by observing the edges of the beds, 
and noting their thickness in making a horizontal section from a 
to b. It is, however, by no means certain that the thicknesses of 
strata observed at the surface are at all the same as those of the 
same beds at considerable depth, neither is it always the case^ 
that the material is the same. What is shale or even clay near 
the surface may be hard slate rock below, what is soft broken 
limestone may be very compact solid marble, and what is 
powdery at the outcrop may be a substance below that would 
require gunpowder to touch. Thus the horizontal section tells 
us of the general nature of the rock after it has long been exposed 
to the action of weather, while the vertical section informs us at 
once of the real nature of the rock under pressure and without 
being changed. 
It is common enough among geologists to give sections in 
which some striking and important fact is distorted ; and this 
seems almost necessary when, as is often the case, it is required 
to limit the section in length and yet represent many facts. 
The scale for horizontal distances must be, for this purpose, very 
