wilh the result that surface streams 

 become aligned along them. Likewise 

 the joint partings direct water moving 

 underground. In limestone, solution 

 proceeds fastest along the joints, and 

 so the general pattern of a cave-system 

 in limestones (such as at Buchan and 

 Jenolan) usually depends on the 

 arrangement of the joints. 



Actually, joints play an important 

 part in the decomposition of rocks, 

 as they permit the access of atmos- 

 pheric and aqueous agents to con- 

 siderable surfaces which would other- 

 wise have not been exposed. In some 

 rocks joints are inconspicuous, hut 



evidence of their presence is usuallv 

 discernible. For example, in a reddish, 

 ferruginous sandstone the positions oi 

 the joints may he shown by bleached 

 bands. Often a space is developed in 

 the rock between the joint surfaces: 

 this is called an open joint or fissure. 

 This space may be filled later with 

 minerals, such as quart/, calcile and 

 limonite. to form thin veins. Some- 

 times valuable ore veins are developed 

 along joints. 



There seem to be several possible 

 causes lor the formation of joints 

 The fractures are a response lo dis- 

 tributed forces (stresses) in rocks, 



Columnar jointing in basalt, known as the "Organ Pipes", near Sydenham. Victoria 



1'holo; Author 



348 



Vict. Nat. Vol. 88 



