488 OVERLYING ROCKS. 



analogous to those just described, that it is un- 

 necessary to repeat them. In certain states of 

 minute admixture, it is, like augit, so easily con- 

 founded with hornblende, that we have still to 

 learn whether hypersthene rock is not of more 

 common occurrence than has yet been sup- 

 posed. Hitherto it has occurred only in my 

 researches, and has been found in Sky and 

 Airdnamurchan. 



Another ingredient in the rocks of the trap 

 family is common felspar ; and it is most re- 

 markable in the porphyritic division, on which it 

 confers the distinguishing character. But it oc- 

 curs also in the syenites, and even in the green- 

 stones, without communicating to them the por- 

 phyritic structure. The structure of the rocks 

 is, in these cases, granitic, and the felspar is dis- 

 tinguished from the compact sort, by its confused 

 crystalline texture. In the same way, it occurs 

 with augit and with hypersthene. Thus there are 

 two subdivisions of all these rocks ; the one set 

 containing the compact and the other the crystal- 

 lized or common felspar ; the external characters 

 of each being strongly distinguished. This is 



