VI N OF R0< 67 



the Mountain Limestone. The reader is referred to 

 Chapters XVI and further information on this 



iubj. 



3. Hardness and Streak* Rocks differ much 

 from each n hardness; even in the same mass 



of rock considerable diversities in this respect may 

 be met uh Hardness is a character of secondary 

 importance, though it may often be usefully employed 

 to distinguish, among the compact rocks, siliceous from 

 softer calcareous masses. Obviously it can only be 

 properly applied to perfectly fresh surfaces, and is 

 suited to homogeneous rather than to comjx>und rocks. 

 he purpose of applying it in the field, a pocket- 

 should be carried, but if the hammer has sharp 

 corners on its cutting edge, these if drawn across the 

 surface of the rock will serve the purpose. The scale 

 rdness employed in mineralogy may be used in 

 ^ rocks. This subject is further referred to in 



"ak is the name given to the powder made when 

 the knife (or file, or diamond) is drawn across the surface 

 of a mineral or rock. Though sometimes useful in 

 ;s not often of much service among rocks. 

 It may now and then be employed to distinguish 

 compact dark bituminous clays or shales from varieties 

 of coal, the former giving a dull brown or grey powder, 

 and the latter a lustrous black streak. In the case of 

 impure calcareous rocks, when little or no efferves- 

 cence is visible in a drop of weak acid placed upon 

 the clean surface, brisk disengagement of carbonic acid 

 may often be produced by dropping the acid over the 





