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JOINTS. 



119 



used, and in those which are blasted. In both cases they Upper 



Gallery, 



guide the quarrymen as to the method by which masses of 

 rocks may be obtained of the largest size, and with the 

 least expenditure of labour. By using the joints as a 

 guide, large blocks of sandstone, limestone, slate, granite, 

 &c, are detached entire; and in cases where great blasting 



Wall-case 46 



operations are conducted (as 



d) 



judicious levels and galleries, in accordance with systems 

 of joints, as much as 118,000 tons of quartz rock have 

 been dislodged and broken up by one explosion. In slate 

 quarries care is taken to blast so that the largest jointed 

 masses are dislodged. These are again broken up along 

 the lines of joints and cleavage by the use of the wedge 

 and mallet, and the rock is thus prepared for the manu- 

 facture of slates. 



Joints and Cleavage. 



■ 



L— -Ironstone, White Stone, Coal-measures, RusseVs 



Hall, Dudley, 



Staffc 



Map 62 



W 



The 



upper and under surfaces are beds. The sides are joints, 

 and the ends ordinary fractures. 



2.— Ironstone, Big vein, Coal-measures, Sirhoicy Iron- 



Monmouthshire. Map 



Contains fossils, Anthra- 



cosia. 



The surfaces of black shale are the top and bottom 



of the bed. The other sides are joints. 



3. — Ironstone, 



thumberland. 



front 



ifi 



rocks, Netherton, Nor- 



of the beds, the other sides are joints forming an irregular 



rhomb. 



4. — Chert, in 



iferous limestone, Matlock 



Derbyshire. Map 82 S. W. The numbered (red) and oppo- 



fracture 



faces of 



The top is an accidental 



5— Felspathic rock, near Toplel 

 coast near St. Minver, Cornwall M 



■forming a rhomb. 



/ Hole, Cave on 

 30. Four joints 



