DEVIATION AND ITS CAUSES 7 



great importance. It is here that we find the greatest 

 advancement in the technique of borehole-survey apparatus. 

 This is significant not only because the proximity of other 

 boreholes greatly increases the possibility of deflection 

 but ignorance of the actual courses of the holes here would 

 give rise to great trouble and expense later on; and perhaps 



Fig. 2. — Course of a full suite of boreholes for a freezing shaft (depths in meters) . 



disaster when encountering the unsolidified gaps between 

 widely isolated frost walls or cementation zones. These 

 possibilities will be apparent from a perusal of Fig. 2, 

 which is an actual survey of the course of such holes previous 

 to sinking operations. 



As the shallower seams and veins are won in the world's 

 mineral fields it is manifest that deep prospecting holes to 



