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characters which seem principally applicable to 



the discrimination of rocks : 



The external configuration of the masses ; or 



their geological forms and disposition. 



The internal structure ; or the concretionary 

 forms which are peculiar to certain rocks and in- 

 dependent of the preceding. 



The texture ; a distinction analogous to that 

 circumstance in the simple minerals. 



The fracture ; a distinction often intimately 

 allied to, or identical with the preceding; but, 

 frequently also, independent of it. 



The hardness ; or the degree in which they 

 resist the action of hard and pointed bodies of dif- 

 ferent kinds. 



The frangibility ; a circumstance distinct from 

 the former, and consisting in thq greater or less 

 facility with which they are broken. 



The lustre ; or the degree in which they re- 

 flect light. 



The degree in which they transmit light ; or 

 the transparency. 



The effect of acids in extricating air. 

 .' The specific gravity. 



ii 



