222 GENERAL REMARKS ON GNEISS. 



be supposed to form a part of gneiss, considered as a geolo- 

 gical formation, and those which ought to be viewed as 

 alternating formations. Perhaps the following criterion 

 may be deemed a just one. Where the interstratified 

 substances follow all its contortions and are traversed by 

 the same granite veins, the whole may be considered 

 as one rock-formation, and may be included in geological 

 description under the common term gneiss. On the con- 

 trary, when the other rocks that accompany it form 

 distinct masses, however slightly confounded at the point 

 of change, are free from the irregularities and veins that 

 attend the gneiss, or are succeeded after such alternation 

 by independent masses of the same rock, or of others 

 usually associated with them, they may be supposed 

 to belong to a different series. 



In the first of these divisions, hornblende schist is by 

 far the most abundant rock, and it appears to predominate 

 chiefly where the gneiss itself contains hornblende as an 

 ingredient. Under this head are ranked the compound 

 schists sometimes known by the name of primitive green- 

 stone. Actinolite schist occurs in a similar manner, but 

 is rare. Micaceous schist must also be ranked with these 

 substances, however difficult it may sometimes be to dis- 

 tinguish it from the schistose varieties of gneiss. Lime- 

 stone also occurs in the same manner, but in large irre- 

 gular masses more frequently than in distinct beds. Lastly, 

 quartz is occasionally found interstratified with gneiss, 

 sometimes simple, at others interspersed with crystals 

 of felspar or with hornblende and felspar both. The 

 circumstances under which it is undistinguishable from 

 granite have already been stated and do not justify the 

 notion that this rock is actually interstratified with it, 

 either as part of the series or as a distinctly alternating 

 substance. 



But the most important part of the geological history of 

 gneiss is that which is included under the second 

 division and which relates to its position with regard 



