fuller] IMPORTANCE OF ACCURATE WELL RECORDS. 15 
On the other hand, deep drilling has been so extensively conducted 
in this country, especially in the East, that there are very few regions 
of any extent that do not have one or more deep borings. These fur- 
nish, where the data have been preserved, our best evidence of under- 
ground conditions, and, combined with surface observations, form a 
basis for predicting the materials to be encountered in new borings. 
Where there are a number of borings they afford accurate data for 
the calculation of dip, which as determining structure becomes an 
all-important factor in oil and gas operations. This is an especially 
important feature in regions where, as in much of Ohio, the rocks are 
buried by hundreds of feet of drift and are nowhere exposed to view 
at the surface. Two wells of known elevations and penetrating a bed 
at known depths are sufficient to establish the dip in such regions with 
greater accuracy than if outcrops were present and served as a basis 
for the determination. With distances and the amount and direction 
of dip known the depth can, as has alread} r been indicated, be readily 
predicted at a given point. Without borings this would often be 
impossible. 
Character of material. — Of almost as much importance as the depth 
of a well is the character of the material penetrated. On it depends 
the nature of the process of drilling employed. In the soft material 
of the Coastal Plain, as in parts of Alabama, Mississippi, and Louisiana, 
driving, jet, and rotary processes, either singty or in combination, are 
used, but none of these are suitable for rock work. If, therefore, a 
very hard bed is to be penetrated another type of outfit must be pro- 
vided. The character of the tools and the rig itself must also be 
adapted to the nature of the material to be penetrated. In rock the 
rate of drilling varies from a few inches in the harder limestones to 
over 100 feet a day in certain shales; hence, in contracting for a well, 
knowledge of the character of the various beds and of their thickness 
is of the greatest value. 
As already intimated, important variations in the character and thick- 
ness of the beds may take place underground without there being at the 
surface any evidence of the changes, and while it is possible to judge, 
to a certain extent, of the character of the beds beneath the surface 
from their more or less distant outcrops, by far the most accurate 
information is that afforded by the borings in the same or adjacent 
regions. 
Water supplies. — When wells are located in valleys, or in thickly 
settled districts, water for boiler use can usually be obtained from 
streams or the wells of near-by houses. In many of the oil fields, 
where the wells are frequently located upon the sides or tops of hills 
or mountains, the question of boiler supplies is important, and small, 
separate wells are often drilled for this supply. Beyond the ability of 
the rock to hold water, as indicated by its texture, little is shown by 
