124 | GEOGNOSY. 

granules there has been a subsequent development of quartz, partly 
round these granules and partly in indefinite layers through the — 
rock. 
Among the varieties of mica-schist may be mentioned, Sericite- 
schist, composed of an aggregate of fine folia of the silky micaceous 
mineral sericite in a: compact honestone-like quartz; Paragonite- 
schist where the mica is the hydrous soda variety, paragonite ; 
Margarodite-schist where the mica is the hydrous form, 
margarodite. 
Mica-schist, together with other schistose rocks, forms exten- 
sive regions in Norway, Scotland, the Alps, and other parts of 
Europe, and vast tracts of the Archean regions of North America. 
It is also found encircling granite masses (Scotland, Ireland, &c.) 
as a metamorphic zone a mile or so broad, which shades away 
into unaltered greywacke or slate outside. In these cases it is 
unquestionably a metamorphosed condition of ordinary sediment- 
ary strata, the change being connected with the extravasation of 
ranite. 
: Though the possession of a fissile structure, showing abundant 
divisional surfaces covered with glistening mica, is characteristic of 
mica-schist, we must distinguish between this structure and that of 
many micaceous sandstones which can be split into thin seams each 
splendent with the sheen of its mica-flakes, A little examination 
will show that in the latter case the mica has not erystallized an 
situ, but exists merely in the form of detached worn scales, which, 
though lying on the same general plain, are not welded into each 
other as in a schist; also that the quartz does not exist in folia but 
in rounded separate grains. 
Gneiss, a schistose aggregate of orthoclase (sometimes also 
oligoclase), quartz, aud mica. It differs from granite chiefly in 
the foliated arrangement of the minerals. The quartz sometimes 
contains abundant liquid cavities, in which liquid carbon dioxide has 
been detected. The relative proportions of the minerais, and the 
manner in which they are grouped with each other, present great 
variations, As a rule, the folia are coarser and the schistose 
character less perfect than in mica-schist. Sometimes the quartz 
lies in tolerably pure bands a foot or even more in thickness, with 
plates of mica scattered through it. These quartz layers may be 
replaced by a crystalline mixture of quartz and felspar, or the 
felspar will take the form of independent lenticular folia, while the 
laminze of mica which lie so abundantly in the rock, give it its fissile — 
structure. Among the accessory minerals, garnet, tourmaline or 
schorl, hornblende, apatite, graphite, pyrites, and magnetite may be 
enumerated. 
Many varieties of gneiss occur, some distinguished by peculiarities" 
of structure, as where the rock is very fissile, or where it becomes 
granular or granitic; others by special minerals, as mica-gneiss, 
