Chlorite- schist  is  a  green  slaty  rock,  in  which  chlorite  is  abun-
dant in  foliated  plates,  usually  blended  with  minute  grains  of
quartz,  or  sometimes  with  felspar  or  mica;  often  associated
with,  and  graduating  into,  gneiss  and  clay-slate.
Quartzite,  or  Quartz  rock,  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  of
other  metamorphic  rocks.  Compact  quartz,  like  that  so  fre*
quently  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  felspar
and  mica.
Crystalline  or  Metamorphic  limestone. — This  hypogene  rock,
called  by  the  earlier  geologists  primary  limestone,  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  resembling  in  colour  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,  felspar,  hornblende,  talc,  chlorite,
garnet,  and  other  minerals.  It  enters  sparingly  into  the  struc-
ture of  the  hypogene  districts  of  Norway,  Sweden,  and  Scotland,
but  is  largely  developed  in  the  Alps.
Origin  of  the  Metamorpbic  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
history  with  an  account  of  the  opinions  entertained  of  their
probable  origin.  At  the  same  time,  it  may  be  well  to  forewarn


564      ORIGIN  OF  THE  METAMOEPHIC  STEATA.    [CH.  xxxm.
