130 GUIDE TO THE AUSTRALIAN MUSEUM. 



though natural, mode of weathering, such as would be the case in a 

 hole under a waterfall, where the process of whirling has been 

 kept on for centuries. 



Tlie different modes of structure are exhibited, namely — 

 granular, lamellar, fibrous, radiate, concretionary, dendritic, 

 earthy, vitreous, &c. These expressions are always used in the 

 description of minerals, and the specimens exhibited afford some 

 points of comparison. 



When a mineral is freshly broken the fracture is sometimes 

 a very good means of determination. In crystallized minerals 

 it is either irregular, or takes place along certain planes, 

 which are called planes of cleavage. Iceland Spar (Calcite) is a 

 good instance of three perfect and easy cleavages. Some crystal- 

 lized minerals, however, exhibit a curved surface when broken, 

 and such fracture is called conchoidal ; the best examples of 

 conchoidal fracture are given by vitreous, and some more or less 

 homogeneous minerals. Ex. : Volcanic Glass, Flint, and Kerosene 

 Shale. 



On handling minerals certain mechanical properties are mani- 

 fested by tests, which also serve to distinguish them. Hardness 

 is thus an important character. In order to find an easy defini- 

 tion of the degree of hardness of a mineral it is compared with 

 some other minerals which are considered as standards, and are 

 known by numbers representing the degree of hardness. The list 

 thus formed is called the scale of hardness. It is arranged as 

 follows : — 



1. Talc. ) 



„' > Scratched by the nail. 



2. Gypsum, j -^ 



3. Calcite. \ , -, ■, 



. T-ii CI ( Scratched by 



i. Fluor bpar. > ^^^^j ^J_ 



5. Apatite. ) 



a 



5. Apatite. 



6. Orthoclase. Scratches glass. 



7. Quartz. 



8. Topaz. 



9. Corundum. 



10. Diamond. The hardest of all known substances. 



