390 Geology. — On the Comtituent Minerals, ^. 



comes schistose, or slaty, and splits into parallel tables. Werner 

 applied the term gneiss to those slaty granites, of which there 

 are many varieties. That kind which is nearest in position to 

 the granite, is generally large grained, its foliation imperfect, 

 and contains occasionally patches of true granite. This kind 

 of gneiss is less fissile than the slaty or schistose varieties. 

 From the parallelism of the layers of gneiss, which often gives 

 it a striped appearance, it has not only been deemed by some a 

 stratified rock, but that this linear regularity is the result of 

 aqueous deposition. We conceive these opinions to be prema- 

 ture, and derived from partial examinations. No doubt this 

 rock often, for considerable distances, presents a stratified ap- 

 pearance, arising altogether from the distinct parallelism of the 

 layers of mica ; but these layers, and thq consequent fissile pro- 

 perty of the rock, may be as reasonably referred to igneous 

 fluidity, since other minerals occur in traps, arranged in a lami- 

 nar way, and giving a fissile tendency to the rocks they are 

 thus found in ; mica itself, too, has been found in veins of trap, 

 holding the same parallelism that it does in gneiss. Sometimes, 

 in gneiss, these layers, after maintaining their linear appear- 

 ance, are gradually lost in amorphous masses ; it is difficult 

 to conceive of the possibility of materials being thus brought 

 together by aqueous deposit ; and the passage of granite into 

 gneiss, which has been proved in such various ways, together 

 with other circumstances, certainly points to a common origin 

 for both granite and gneiss. 



Some of the varieties of slaty gneiss pass into Mica slate. 

 When quartz and mica, two of the constituents of granite, are 

 well combined together, they form this rock, which splits easily 

 into tables. Sometimes the plates of mica are so large, as to 

 give it a coarse character ; at other times, the quartz and mica 

 are so intimately combined, that the mica slate is very fine. It 

 usually has a light gray colour, derived from the black and 

 •w>hite mixture of its constituents, but has frequently a yellowish 

 and greenish hue, with a pearly lustre. Sometimes beds of this 

 mineral are found alternating with gneiss, and it is frequently 

 traversed by seams of quartz, running parallel with the plane 

 of its own bed. These seams increase in thickness as they de- 

 scend, indicating their inferior origin. Mica slate is usually found 

 associated, more or less, with the granite and gneiss of this con- 



