76 



GRANITIC GNEISS. 



unite with them chemically." After enumerating 

 several instances where granite is found protru- 

 ding through the superincumbent rocks. Prof. H. 

 proceeds to sum up his reasons for beheving this 

 rock to be of igneous origin ; which, although we 

 purposely design in the present work to abstain 

 from theoretical geology, are yet so conclusive as 

 to merit particular mention. These are, 1. The in- 

 clined position of the older stratified rocks : 2. The 

 manner in which it is intruded among the stratified 

 rocks : 3. The mechanical effects which it appears 

 to have exerted upon the stratified rocks in its im- 

 mediate vicinity : 4. Its mechanical effects upon 

 the surrounding strata : 5. Its crystalline structure, 

 and the numerous crystallizations of other substan- 

 ces that have taken place in it. 



Gneiss. 



Gneiss is composed of the same minerals as gran- 

 ite, namely, quartz^ feldspar^ and mica^ arranged in 

 distinct layers ; indeed, it may be called a stratified 

 granite. Granite frequently passes into gneiss by 

 an almost imperceptible gradation ; where the quan- 

 tity of feldspar decreases, and the crystals or grains 

 become smaller, if the mica increases in quantity 

 and is arranged in layers, the rock loses the massive 

 structure and becomes slaty, we have then a true 

 gneiss. But if the reverse of this happen, the quan- 

 tity of feldspar increases and the mica diminishes, 

 the rock loses the slaty structure and becomes mass- 

 ive, and we have granite again. Gneiss rock con- 

 tains beds of crystalline limestone and of hornblende 

 rock ; also most of the metallic ores, garnet, epidote, 

 <fec. The declivities of granite mountains are often 

 covered by rocks of gneiss; where it forms mount- 

 ains, they are not steep and broken as those of 

 granite, and the summits are for the most part 

 rounded. 



