DEVIATION AND ITS CAUSES 17 



The hardness of minerals is fairly constant but of rocks 

 this is not the case. This is due to the fact that minerals 

 have a more definite and rigid chemical constitution than 

 rocks, since the latter are aggregations of minerals. The 

 minerals in rocks being in any proportions between certain 

 arbitrary limits the hardness of a particular rock varies with 

 its type, i.e., the percentage of its dominant mineral. 



c. Inclined strata especially rapid changes in the inclina- 

 tion as in boring through sharp unconformities, domes, 

 folds and thrusts. The tool tends to follow the dip at 

 the contact. (However, this is not a rigid statement.)^ 

 If we are dealing with the percussive system we must bore 

 with short strokes so that the cutting tool meets a cleaner 

 face since the rinsing water can better deal with the debris. 

 With no rinsing system the hole must be sludge pumped 

 often so that the direction of impact is in the prolonged 

 line of the rods. If this is not done the chisel will nurse 

 the dip. In the rotary system of boring these difficulties 

 are often almost insurmountable. 



Other geological causes of deviation of a drill hole may 

 be: 



1. Bowlders, concretions and dykes. 



2. Faults, thrusts and unconformities. 



3. Caving and movement of strata in the uncased part 

 of the hole. 



4. General earth movement. 



d. Lack of Rigidity in the Rods. — Even in the tightest 

 joints the slightest joint play will initiate curvature with 

 straight rods, just as railway curves can be made entirely 

 of straight rails. 



e. The Proximity of Other Boreholes. — In boring by 

 percussive methods, for instance in the freezing process 

 for shafts, the ground is disturbed by the continual shock 

 of the tool so that new holes put down near by tend to 

 deviate into the zone of least resistance. Again any iron 

 such as parts of old tools or casing in the old hole will 

 accentuate the deflection. This of course applies also 



^ Kitchen, op. cit., p. 100. 



