60 DEEP BOREHOLE SURVEYS AND PROBLEMS 



core below. Between Ai and A 2 is a compass box C of 

 non-magnetic material with a compass D and a clockwork 

 mechanism E screwed on tight, which has been set to 

 operate at a predetermined time. The plastic mass having 

 been lowered over the core stub and allowed to harden, 

 and the needle arrested, the apparatus is raised and the 

 position noted. The core is now lengthened by the usual 

 coring process, wrenched off and raised to bank. Here it is 

 fitted to the impression in the cast and turned with the com- 

 pass until the needle plays in the position previously noted. 

 The dip and strike can now be read. 



The method appears theoretically to be well suited to its 

 purpose and it has the advantage of increased protection 

 for the compass and clockwork mechanism, and also the 

 time taken in insertion and employment is shorter than in 

 previous methods. However, its success depends on many 

 factors which preclude its adoption in general practice. 

 Thus we have the following disadvantages: 



1. Mud and cavings prevent good impressions. 



2. A flat upper fracture on the core surface is more suited 

 to the process than inclined ones, because very inclined 

 wrench faces prevent good impressions. 



3. On inclined core faces tube A 2 is likely to slip and ren- 

 der results faulty. 



4. The core must be solid and fast; this is not possible 

 in shales, schists, etc. 



5. On fitting the mold aboveground the core must have 

 been raised in exactly the same position as it had when the 

 mold was taken, and this is almost impossible. 



6. The minimum size of core is 5 to 6 cm., otherwise the 

 impression is not clearly recognizable. 



7. Even with all the above conditions fulfilled, taking 

 the mold, lengthening the stub, wrenching it off and raising 

 it occupies too much time. 



Koebrich's Apparatus. — In this method the position 

 of the compass with respect to the core is ascertained 

 by means of a clearly cut mark on the top face of the core 

 with the aid of the apparatus shown in Figs. 1 to 6 (Plate 



