ON THE DIP OF THE UNDERGROUND PALAEOZOIC ROCKS. 



443 



drilled, and the diamonds, being harder than any other known substance, 

 cut their way through, forming an annular groove which becomes deeper 

 and deeper as the crown is slowly lowered. A cylindrical core of un- 

 detached rock is thus left standing in the middle (tig. 2), and is received in a 

 long tube of a somewhat greater internal diameter than the crown which 

 it surmounts. To the closed top of this core-tube is attached a column 

 of tubular rods, by which a I'apid rotary motion is transmitted from 

 machinery above, and down which water is pumped, with the double 

 purpose of keeping the drill cool and of bringing up the abraded material. 

 To admit of the passage of the water from the inside to the outside of the 

 crown, grooves are formed across the cutting face, and through these the 

 water is forced, carrying with it the debris cut away by the diamonds. 

 The water then passes upward outside the core- tube to the top of the 

 hole, bringing with it the lighter of the particles. Above the core-tube, 

 and enclosing the lower end of the suspending rods (fig. 3), there is an open 

 topped tube of a few feet in length, into which the heavier matter falls, 

 and this sediment tube is emptied when the crown is raised to the surface. 

 Before the core can be brought up from the bottom it must be detached 

 from its bed. This is usually effected by friction during the revolution of 



Fig. 2. 



Fig. 1. 



Crowrv 



-Duimonds 



Scale, 1 in. to 1 ft. 



Scale, 1 in. to 2 ft. 



the crown in boring, the inner surface grasping the core with sufficient 

 force to break it off. The block then falls out of the perpendicular, and 

 when the boring-rods are drawn up, it is generally found with its lower 

 end resting on the intei-nal shoulder of the crown. Most frequently, the 

 fracture occurs at a bed or other joint in the rock. Should the pillar 

 remain firmly fixed after it has attained the full height of the core-tube, 

 one of the various appliances known as core-extractors is lowered. This 

 firmly clips the core at the bottom, and on the application of adequate 

 lifting power to the rods, the piece is broken oflT and can be withdrawn. 

 In this way, solid columns, 30 feet in length, and 16 inches in diameter, 

 were obtained. 



The great advantage of the system is, that substantial specimens of the 

 materials bored through can be obtained, and their relative positions 

 actually seen. No room is left for doubt as to the depths and thicknesses 

 of the various strata, and since the hole is put down perfectly vertical, 

 there is no difficulty in ascertaining the exact angle of the dip if it is 

 steep enough to be appreciable in the core. On the other hand, the rota- 

 tion of the apparatus precludes all knowledge of the direction of dip by 

 mere inspection, and special means have to be devised for ascertaining this 

 particular. 



