MINERAL INDUSTRIES. 
23 
The structural conditions to which reference has been made may 
or may not show at the surface. Thus a country that is monot¬ 
onously level may in fact have pronounced folding- of the rock be¬ 
neath the surface. Conversely a country may be very hilly owing 
to surface erosion and yet having underlying formations that pre¬ 
sent few if any of the structural features favorable to oil. One 
cannot, therefore, select a hill or ridge as indicating the location 
of a fold or a dome. It is true, however, that ridges and hills may 
indicate structural features beneath the surface, and hence should 
receive consideration with respect to oil possibilities. 
A study of the geology of a region frequently affords important 
information as to the most favorable place at which to locate oil 
wells. These studies, however, must be made in detail and should 
cover large areas, so as properly to correlate the structural features 
that are observed. That which is especially needed to further this 
work in Florida is first of all accurate topographic maps by which 
all surface features may be definitely located, and exact well records 
by which the location, depth and thickness of formations may be 
determined. In connection with the well records it is to be noted 
also that the records should be based on the samples of the drillings 
taken at frequent intervals as often at least as every ten feet. The 
driller’s log and notes are of value, but they should, be supplemented 
in all cases by the actual rock samples. 
SURFACE INDICATIONS. 
In a locality where oil occurs surface indications may or may 
not exist. Those which are found frequently are in the form of 
the escape of oil through leaks into springs, streams or other open¬ 
ings in the earth. The channels of streams not infrequently cut 
into a rock formation that contains oil. In this case oil may show 
on the stream, or the excess of oil having passed away may be de¬ 
tected only by the stain in the rock or by a test by which the re¬ 
maining oil is dissolved from the rock and is thus detected. All 
such surface indications which seem possibly to indicate the location 
of oil or gas should receive careful investigations. 
DEPTH OF WELLS. 
The depth at which oil and gas are found varies from a few 
feet to such depth as is practicable to drill. A small supply has 
frequently been obtained from very shallow wells, and large sup- 
