THE GNEISSES 143 



gneisses is essentially the same as that of the granites, from 

 which they differ only in structure and origin. They, how- 

 ever, present a greater variety and abundance of accessory 

 minerals, chief among which may be mentioned (besides those 

 of the mica, hornblende, or pyroxene group) garnet, tourmaline, 

 beryl, sphene, apatite, zircon, cordierite, pyrite, and graphite. 

 Structure. — Structurally the gneisses are holocrystalline gran- 

 ular rocks, as are the granites, but they differ in that the various 



-O' 



Fig. 15. — Microstructure of gneiss, showing at the point a broken feldspars. 



constituents are arranged in approximately parallel bands or 

 layers, as shown in PL 12. 



In width and texture these bands vary indefinitely. It is 

 common to find bands of coarsely crystalline quartz several 

 inches in width, alternating with others of feldspar, or feld- 

 spar, quartz, and mica, or hornblende. A lenticular structure 

 is common, produced by lens-shaped aggregates of quartz or 

 feldspar, about and around which are bent the hornblendes or 

 mica lamince. The rocks vary from finely and evenly fissile 

 through all grades of coarseness, and become at time so mas- 

 sive as to be indistinguishable in the hand specimens from 

 granixes. 



Colors. — Like the granites, the gneisses are all shades of gray, 

 greenish, pink, or red. 



Geological Age and Mode of Occurrence. — The true gneisses 

 are among the oldest crystalline rocks, and have been considered 



