CRYSTALLIZED MARBLES. 189 



forms the highest, and at the same time the central part 

 of mountain-groups. The next rock, in point of anti- 

 quity, or that which rests immediately upon the granite^ 

 is gneiss, which has a distinct slaty structure, is stratified, 

 and, like granite, is composed of felspar, quartz, and 

 mica. It alternates with the newer portions of the gra- 

 nite, and sometimes cotemporaneous veins of the one 

 rock shoot into masses of the other. It contains subor- 

 dinate formations of granite, porphyry, syenite, trap, 

 quartz, limestone, and conglomerated gneiss. The next 

 rock in the series is mica-slate, which rests upon the 

 gneiss. It is composed of quartz and mica, and has a 

 distinct sfaty structure, and is stratified. It alternates 

 with gneiss, and contains various subordinate formations, 

 as granite, porphyry, syenite, trap, quartz, serpentine, 

 limestone, and conglomerated mica-slate. It is often 

 traversed by cotemporaneous veins, from the smallest 

 discernible magnitude to many yards in width. The 

 mica-slate is succeeded by clay-slate, which rests upon 

 it, and sometimes alternates with it. It differs from 

 mica-slate, gneiss, and granite, in its composition, being 

 in general a simple rock ; and in some instances princi- 

 pally composed of mica, in others to all appearance of 

 felspar. Besides granite, porphyry, trap, syenite, lime- 

 stone, serpentine, conglomerated clay-slate,* quartz, it 

 also contains the following formations; flinty-slate, 

 whet-slate, talk-slate, alum-slate, and drawing-slate. The 

 calcarious rocks mentioned by Cuvier. as resting upon 

 the slate, do not belong to this class ; they are transi- 



, 



* The primitive conglomerated rocks, mentioned as above, as occur- 

 ring in gneiss, mica-slate, and clay-slate, are sometimes named grey- 

 wacke, 



