174 



specimen of a sheared pebble is, perhaps, that from the All 

 !N'ations Mine, Drake, one portion of the pebble being displaced 

 very considerably. 



At Bushy Hill, Cooma, and many other places, quartz- 

 porphyries exist which have been converted into a slaty rock by 

 intense crushing within the Earth's Crust. Specimens of 

 crushed and uncrushed j)orphyry, together Avith a series of 

 photographs showing the effect of crushing upon rocks, are 

 exhibited in 112. 



At Broken Hill great crushing has taken place. In this way 

 the large crystals of garnets and quartz in schist have been drawn 

 out somewhat, and remain as eyes, while the rest of the rock has 

 flowed round them under the influence of the pressure. 



Contortion. — -Quartz-veins, clay-shale, &c., have been crumpled 

 by lateral pressure. This is shown by specimens in cases 112, 

 113, 138 (Plate IX). 



Cleavage, case 138. — A piece of Cambrian slate from South 

 Australia shows a series of parallel cracks which have been 

 produced by earth-strain. The cracks are filled iii with calcite, 

 and occur in certain bands only of the rock. 



The meaning of cleavarje is well illustrated by a piece of 

 elate from South Australia. The original bedding-planes are 

 indicated by differently-coloured bauds. The rock, however, no 

 longer splits along these surfaces, but along a new set of parting- 

 planes, one of which coxastitutes the face of the specimen. This 

 second set of parting-planes has been set up by the pressure to 

 which the rock has been subjected since its consolidation. 



Faulting. — Hand- specimens sometimes show on a small scale 

 the displacement known to the miner as " faulting." In case 138 

 will be found sandstone, shale, limestone and clayslate that have 

 been faulted. The polished marble from Fernbrook and a pebble 

 of chert show a double fracture, producing a trough- fault. There 

 is also a quartz-vein that has been faulted by two others. 



Photographs. Photographs illustrating structural geology are hung on the 

 north wall ; others are mounted in an album facing the entrance. 



