148 GNEISS. 



The most common substance in the series of con- 

 torted gneiss, is hornblende schist, which even seems 

 essential to the granitic varieties containing horn- 

 blende, since it is never absent ; while if it also occurs 

 in others, it is rarely or ever in the same abundance. 

 Occasionally indeed, the hornblende schist so far ex- 

 ceeds the gneiss in quantity, that this latter might be 

 held the subordinate rock. Analogous to this is acti- 

 nolite schist, differing from it only in the slight dis- 

 tinction which exists between the two minerals, but, 

 comparatively, of very rare occurrence. In a similar 

 manner, there are found beds of micaceous schist ; 

 though it is sometimes difficult to distinguish the 

 schistose gneiss from this rock. The distinction is 

 obvious when it occurs among the granitic varieties ; 

 though this is rather a rare incident. If chlorite schist 

 also occurs in the same manner, the analogy of these 

 rocks is such that I need not dwell on it. Limestone 

 is sometimes also found included in the gneiss series, 

 'in limited flat beds, and in large irregular concretions, 

 or immense nodules. Beds of quartz are equally rare ; 

 being sometimes simple, at others, interspersed with 

 irregular crystals of felspar ; while often so circum- 

 stanced that it is difficult to determine whether they 

 are beds or veins. A granitic compound, without any 

 marks of foliation, resembling some of the syenites of 

 the trap family, and consisting of an uniform mix- 

 ture of quartz and felspar interspersed with acicular 

 crystals of hornblende, also occurs in Sky and in other 

 places, yet rarely. If it is probable that many other 

 peculiar occasional compounds occur in the same situ- 

 ations, as yet unnoticed, I shall point out but one 

 more, from having perceived that they are sources of 

 difficulty to students, or else induce them to suppose 

 that they had discovered a new rock in the general 



