16 RECORD OF DEEP WELL DRILLING FOR 1904. [rim, 264. 
the surface outcrop. Nothing as to crevices, joints, local sandy beds, 
etc., on which so much depends, is indicated. All this can be finally 
determined only by the drill. In this case a single boring, unless in 
the immediate vicinity of the proposed new well, will not afford an 
absolute indication of the conditions, but from a considerable number 
sunk in the same kind of rock an average is obtained which shows the 
predominating conditions characterizing the bed. This is especially 
true of the quality of the water, which will vary greatly in different 
localities, often without any reference to conditions discoverable at the 
surface. 
( 'asing off water. — Among the factors affecting the sinking of a well 
is the occurrence of water in quantities necessitating casing it off. 
This is of importance in determining the amount of casing necessary 
and its cost, and in determining the size of the original hole in case it 
is anticipated that several horizons at some distance from one another, 
each of which may involve a reduction of the size of the casing, will 
be encountered. As in the case of the water supplies discussed in the 
preceding paragraph, the most satisfactory information is that fur- 
nished by the records of adjacent wells. 
Casing off "cave" rock. In many of the older wells in the softer 
rocks, especially in shales, much trouble was experienced by the walls 
of the hole caving and stopping up the bore. This difficulty will be 
recalled by those familiar with the history of the old Dunkard Creek 
Held, in southwestern Pennsylvania, where many of the wells sunk 
without casing were ruined by the caving of the walls. Even now that 
the caving is easily preventable by proper casing, a knowledge of the 
depths and thicknesses of troublesome beds is of importance in deter- 
mining in advance the questions relating to casing and its effect on the 
cost of the well. This information is best furnished by records, as in 
the case of the water horizons already discussed. 
Limits of depth. — Nothing can be further from the truth than the 
common fallacy that oil, gas, water, or whatever the driller may be 
in search of, can be had if the hole is only drilled deep enough. Ii 
unaltered porous rocks oil and gas may occur, within certain limits, 
regardless of depth; but all rocks, of whatever character, if buriec 
deep enough, especially if subjected to the powerful compressioi 
incident to the formation of mountains, become altered through th< 
influence of heat and pressure, often into more or less crystalline 
masses. In such rocks the volatile substances, such as the hydrocar 
bons, are expelled and it is useless to look in them for deposits of oi 
or gas. It is also useless to seek for oil in rocks of igneous origin— 
that is, those which have once been molten — for the conditions of thei 
formation are such as to preclude the formation or retention of oil o 
similar substances. Nevertheless, wells have not infrequently beei 
