486 OVERLYING ROCKS. 



as to form, with the leading- ingredient, a com- 

 pound or syenitic base. 



The next mineral which forms an ingredient 

 in these rocks is augit, which, under different as- 

 pects, is of very common occurrence ; much 

 more so than has been imagined, since it has, as 

 I have already remarked, been frequently mis- 

 taken for hornblende. It presents two varieties 

 of colour, dark green and black ; the latter being 

 the most common and the most difficult to recog- 

 nize : the former can scarcely be mistaken. In 

 many cases, the crystals, though indefinite, are 

 so large, that there is no difficulty in determin- 

 ing the presence of the mineral ; when very 

 minute, that difficulty increases ; although, with 

 due attention, it is generally possible to make the 

 distinction. Like hornblende, it is found, in the 

 rocks where it enters, combined with different 

 kinds, both of common and of compact felspar ; 

 and it is most obscure whenever it is accompa- 

 nied with the greenish or dark compact varieties, 

 In Rum, it is united to the glassy variety ; and 

 the distinction is in this case easily made, as 



