IIAP. HARDNESS OF ROCKS 919 



Talc. 



- 



.Idle. 



9. Comodum (Emcfy). 



ia 



$. A) 



A rock which am be easily scratched with the finger- 

 nail, like many chloride-schists, may be said to have one 

 degree of hardness, < r 1 i i . rocks possessing the hard 

 ness of rock-salt (a) can be less readily sera: : the 



finger-nail The |*ocket kmfc easily marks a limestone 

 vstal of calcite (3), which, on the other hand, re- 

 sists the finger-nail ; a little more pressure is required to 

 mark a crystal of fluor-spar (4), and still more one of 

 apatite (5). Hocks possessing the sixth degree of hard- 

 ness can be scratched with the knife with difficulty, while 

 they present greater hardness than about 6) degrees 

 they resist the knife and even turn its edge, or take a 

 streak of steel Hence as rocks of this resisting power 

 are almost always siliceous, the application of the knife 

 furnishes a convenient means of discriminating th 



Obviously in trying the hardness of rocks we must see 

 that we are in possession of fresh undccomposed surfaces. 

 The effect of weathering is generally to loosen the texture 

 of a rock, and make it yield more readily to abrasion, and 

 as the influence of this superficial decomposition may 

 extend for several feet or many yards into a rock, it is not 

 always practicable to obtain a specimen sufficiently fresh 

 to admit of the application of this test. It must be 

 remembered that the hardness of a rock, even when quite 

 fresh, may appear less than from the nature of its com- 

 ponent minerals we might expect it to be. The pressure 

 of the knife bruises and cuts through the edges of 



