CORE ORIENTATION 63 



worthiness of the results. The apparatus is durable and 

 simple and will withstand considerable water pressure, 

 but the difficulties of clockwork devices are here only 

 ameliorated, not prevented. It has many disadvantages : 



1. In friable marls, medium sandstones, weakly cemented 

 conglomerates, etc., it is useless because here we get neither 

 core nor marking satisfactory. 



2. A second blow, on account of the rod tor- 

 sion, would mean a turn of the chisel and a com- 

 plication of the orientation. 



3. Much time is used up in altering the tools for 

 marking, coring, etc. This often takes 6 to 8 hr. 



MacGeorge's Core Orientation Method. 

 MacGeorge (1884) was the first to appreciate the 

 importance of obtaining the inclination of the 

 core at the time of orientation. He used a brass 

 tube set eccentrically, and furnished with a bell 

 mouth below, the office of which was to receive 

 the extremity of any piece of core left standing 

 at the bottom of the bore, and, as the apparatus 

 is forced down, to press on one side and break 

 off the piece of core. 



A plummet a (Fig. 33) is suspended midway g^^ 

 in one or more phials of warmed gelatine h in ^^ 

 suitable containers in the core catcher. The 

 whole apparatus being now left unmoved for 2 or 3 hr., 

 until the fluid in the phials has cooled and set, is 

 withdrawn, and the core extractor unscrewed. The 

 phial of liquid gelatine is firmly grasped and kept in the 

 same relative position as the core in the borehole. 

 The phial, by means of its internal indications, will 

 enable the piece of core to be replaced in its natural 

 position for observation, and thus there may be readily 

 ascertained (by examination of the markings) the true dip 

 and strike of the strata, or the underlie and bearing of the 

 reef, of which it originally formed a part. We shall discuss 

 the method further when dealing with MacGeorge's devia- 

 tion device later. 



