METAMORPHIC ROCKS. 
579 
sometimes appearing to constitute the whole mass. Beds of 
pure quartz also occur in this formation. In some districts, 
garnets in regular twelve-sided crystals form an integrant 
part of mica-schist. This rock passes by insensible grada¬ 
tions into clay-slate. 
Clay-date — Argillaceous Schist — Argillite. — This rock 
sometimes resembles an indurated clay or shale. It is for 
the most part extremely fissile, often affording good roofing- 
slate. Occasionally it derives a shining and silky lustre 
from the minute particles of mica or talc which it contains. 
It varies from greenish or bluish-gray to a lead color; and it 
may be said of this, more than of any other schist, that it is 
common to the metamorphic and fossiliferous series, for some 
clay-slates taken from each division would not be distin¬ 
guishable by mineral characters alone. It is not uncommon 
to meet with an argillaceous rock having the same composi¬ 
tion, without the slaty cleavage, which may be called argillite. 
Chlorite schist is a green slaty rock, in which chlorite is 
abundant in foliated plates, usually blended with minute 
grains of quartz, or sometimes with feldspar or mica; often 
associated with, and graduating into, gneiss and clay-slate. 
Quartzite.^ or Quartz Roch^ is an aggregate of grains of 
quartz which are either in minute crystals, or in many cases 
slightly rounded, occurring in regular strata, associated with 
gneiss or other metamorphic rocks. Compact quartz, like 
that so frequently found in veins, is also found together with 
granular quartzite. Both of these alternate with gneiss or 
mica-schist, or pass into those rocks by the addition of mica, 
or of feldspar and mica. 
Crystalline or Metamorphic Limestone .—This hypogene 
rock, called by the earlier geologists primary limestoyie., is 
sometimes a white crystalline granular marble, which when 
in thick beds can be used in sculpture; but more frequently 
it occurs in thin beds, forming a foliated schist much resem¬ 
bling in color and arrangement certain varieties of gneiss and 
mica-schist. When it alternates with these rocks, it often 
contains some crystals of mica, and occasionally quartz, feld¬ 
spar, hornblende, talc, chlorite, garnet, and other minerals. 
It enters sparingly into the structure of the hypogene dis¬ 
tricts of Norway, Sweden, and Scotland, but is largely de¬ 
veloped in the Alps. 
Origin of the Metamorphic Strata. —Having said thus much 
of the mineral composition of the metamorphic rocks, I may 
combine what remains to be said of their structure and his¬ 
tory with an account of the opinions entertained of their 
probable origin. At the same time, it may be well to fore- 
