258 GNEISS. 



FIRST SUBDIVISION. Granitic. 

 Resembling- granite, i. e. large grained and 

 often losing the foliated structure. 



A. Quartz, felspar, mica. 



B. Quartz, felspar, hornblende. 



C. Quartz, felspar, mica, hornblende. 



These varieties abound in Lewis, Harris, and 

 the associated islands. They sometimes contain 

 superadded crystals of felspar, thus forming the 

 porphyritic gneiss of some authors. 



SECOND SUBDIVISION. -SckistOSC. 



The structure foliated, like that of micaceous 

 schist, or granular with a slight laminar tendency, 

 like that of quartz rock : fine grained. 



A. White felspar and quartz in minute grains in- 

 timately mixed, with rare scales of mica. The position 

 of the mica determines the foliated structure, which is 

 however often so very indistinct that the specimens can 

 scarcely be distinguished from quartz rock. 



B. The mixture of felspar and quartz finely gra- 

 nular as in the former, but the mica so abundant as to 

 cause it to resemble certain varieties of a micaceous 

 schist. 



C. The mica so abundant as to form distinct con- 

 tinuous laminae; and as in these cases it is often very 



