71 



CHAPTER VI. 



ON GNEISS AND MICA-SLATE, AND THE ROCKS WHICH ARE ASSO- 

 CIATED WITH THEM. 



On the Passage of Granite into Gneiss. — Gneiss and Granit vein6. — Mica-Slate. — 

 Formation of Gneiss and Mica-Slate. — Talcous Slate, and Chlorite Slate. — 

 Crystalline Limestone denominated Primary, occurs both in Primary and Sec- 

 ondary Mountains. — Formation of Limestone and Coral Islands by Animal Se- 

 cretion. — Dolomite, or Alpine Magnesian Limestone. — Serpentine and Ollite, 

 orPotstone. — Euphotide or Saussurite the hardest and heaviest of Rocks. — Trap 

 Rocks changed to Serpentine. — Eurite or White Stone. — Primary Porphyry a 

 Mode of Granite, — Recurrence of the same Rocks in Rock Formations of dif- 

 ferent Epochs. 



The principal primary rocks, enumerated with granite in the pre- 

 ceding chapter, were Gneiss and Mica-slate. With these, certain 

 rocks are frequently associated, and are therefore regarded as pri- 

 mary ; for, where one rock occurs imbedded in another, it is evi- 

 dent that the enclosed rock must be as ancient as the rock which en- 

 folds it, unless the imbedded rock has been subsequently protruded 

 within more ancient rocks, as is the case with some volcanic or trap 

 rocks. 



Gneiss received its name from the German miners ; according to 

 Mr. Jameson, the decomposed stone on the sides of some metallic 

 veins was first so called ; but Werner designated by this term a 

 schistose or slaty granite, abounding in mica. Granite frequently 

 passes into gneiss by an almost imperceptible gradation : where the 

 quantity of felspar decreases, and the crystals or grains become smal- 

 ler, if the mica increases in quantity, and is arranged in layers, the 

 rock loses the massive structure, and becomes schistose ; we have 

 then a true gneiss. By the reverse of this process, if the quantity 

 of felspar increases, and the mica diminishes, the rock loses the 

 schistose structure and becomes massive, and we have granite again. 

 Some geologists call this secondary granite ; but the upper and low- 

 er granite, and the gneiss, are in this instance, but different modes of 

 the same rock. 



The granite of the Alps, which Saussure calls granit veine, is 

 properly an incipient state of gneiss-: the mica is arranged in thin 

 parallel laminae varying in distance from each other; when they ap- 

 proach very near, they form what in hand specimens is called true 

 gneiss. When the parallel layers of mica are at some distance from 

 each other, they give a striped appearance to the rock. Laminae of 

 quartz, of considerable thickness, sometimes separate the felspar 

 from the mica, and occasionally, masses of quartz are imbedded in 

 gneiss. When the mica becomes very abundant, and the other con- 

 stituent parts are small in size and quantity, gneiss passes into mica- 



