250 GNEISS. 



as already noticed, is hornblende schist; but 

 quartz rock and micaceous schist are also not 

 uncommon ; and, though much less frequent, 

 argillaceous schist is known to occur in a similar 

 manner. Such beds have been called subordi- 

 nate; a distinction, the propriety of which, how- 

 ever questionable it may be, is not a subject for 

 discussion in this place ; although, in geological 

 descriptions, it is often, not only convenient, but 

 necessary, to consider a series of this nature as 

 gneiss, and to omit all notice of these inferior 

 strata. 



The dimensions of the strata of gneiss are 

 extremely various. They are generally very 

 considerable where they do not alternate with 

 the other strata already mentioned ; but where 

 hornblende schist, in particular, abounds, they 

 are often very thin. Where they are not sepa- 

 rated by the intervention of these and similar 

 strata, the distinctions between the different beds 

 are generally formed by some change in the pro- 

 portions of the ingredients, and principally by 

 variations in those of the hornblende or the 

 mica. 



