228 SYNOPSIS OF GNEISS. 



SYNOPSIS OF GNEISS. 



FIRST DIVISION. 



OF regular composition, containing at least three of the 

 four minerals, quartz, felspar, mica and hornblende. 



FIRST SUBDIVISION. Granitic. 



Resembling granite, i. e. large grained and often losing 

 the foliated structure. 



A. Quartz, felspar, mica. 



B. Quartz, felspar, hornblende. 



C. Quart/, felspar, mica, hornblende. 



These varieties abound in the Long Island. They some- 

 times contain superadded crystals of felspar, thus forming 

 the porphyritic gneiss of some authors. 



SECOND SUBDIVISION. Schistose. 



The structure laminar, like that of micaceous schist, 

 or granular with a slight laminar tendency, like that of 

 quartz rock: finegrained. 



A. White felspar and quartz in minute grains intimately 



mixed, with rare scales of mica. The position of the 

 mica determines the foliated structure, which is how- 

 ever often so very indistinct that the specimens can 

 scarcely be distinguished from quartz rock. 



B. The mixture of felspar and quartz finely granular as 



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

 it to resemble certain varieties of micaceous schist. 



C. The mica so abundant as to form distinct continuous 



laminse ; and as in these cases it is often very difficult 

 to distinguish the felspar from the quartz, the specimens 

 are easily confounded with common micaceous schist. 



