88  PRE-CAMBRIAN   GEOLOGY   OF    NORTH   AMERICA. 
line  limestone,  all  being  rather  similar  to  those  of  the  Upper  Lauren- 
tian  terrane.  These  norites,  which  have  sometimes  been  designated 
by  the  name  gabbro,  must  not  be  confounded  with  the  very  distinct 
gabbros  of  the  Huronian  terrane,  nor  with  certain  plutonic  rocks  to 
which  they  bear  mineralogical  resemblances.  The  facies  of  the  no- 
rites  serves  to  distinguish  them. 
(3)  Arvonian.  This  terrane  is  composed  in  large  part  of  petro- 
siliceous  rocks  which  pass  into  the  state  of  quartziferous  porphyry, 
with  which,  however,  certain  amphibolic  rocks  are  intercalated,  as 
well  as  sericitic  schists,  quartzites,  oxides  of  iron,  and  more  rarely 
crystalline  limestone.  This  terrane,  indicated  for  the  first  time  by 
Hicks,  in  1878,  in  Wales,  is  regarded  by  Charles  Hitchcock  as  form- 
ing in-  North  America  the  lower  part  of  the  Huronian  terrane. 
(4)  Huronian.  This  name  was  given  by  the  author  in  1855  to  a  ter- 
rane already  recognized  in  North  America,  where  it  rests  unconform- 
ably  either  on  Laurentian  gneisses  or  on  Arvonian  hornstones.  It 
comprises,  besides  quartzose,  epidotic,  chloritic,  and  calcareous  schists, 
masses  of  serpentine  and  lherzolite,  as  well  as  euphotides,  which  repre- 
sent in  this  terrane  the  norites  of  the  Norian  terrane,  with  which  they 
are  sometimes  confounded  under  the  common  name  of  gabbro. 
This  terrane  predominates  in  the  Alps,  where  it  forms  the  series  of 
green  stones  (pietre  verdi). 
(5)  Montalban.  The  studies  of  von  Hauer,  published  in  1868,  on 
the  eastern  Alps,  and  those  of  Gerlach  on  the  western  Alps,  published 
the  year  following,  agree  in  recognizing  in  these  regions  two  gneissic 
terranes,  that  is  to  say,  an  old  or  central  gneiss  and  a  young  or  recent 
gneiss;  the  latter,  which  is  very  distinct  from  the  old  gneiss  from  a 
petrographic  point  of  view,  being  accompanied  by  micaceous  and  am- 
phibolic schists.  The  studies  of  Gastaldi,  published  in  1871,  and 
those  of  Neri,  published  in  1874,  while  confirming  Hauer  and  Ger- 
lach's  results,  have  furnished  more  details  on  these  terranes  and  their 
lithological  characters.  It  is  proper  to  remark  here  that  all  these  ob- 
servers seem  to  be  agreed  in  placing  the  horizon  of  the  greenstones 
(Huronian)  between  the  old  gneiss  (Laurentian)  and  the  young- 
gneiss. 
Before  he  had  knowledge  of  the  first  observations  of  these  scientists, 
the  author,  in  accordance  with  his  own  studies  in  North  America,  was 
led  to  identical  conclusions,  and  in  1870  he  announced  the  existence  of 
a  series  of  young  gneisses  quite  distinct  from  the  old  gneisses  and 
accompanied  by  crystalline  limestones  and  by  micaceous  and  amphi- 
bolic schists.  To  this  terrane,  in  view  of  its  great  development  in  the 
White  Mountains  of  New  Hampshire,  he  gave  in  1871  the  name 
Montalban.  This  series,  for  the  rest,  appears  identical  with  the 
young  gneiss  of  the  Alps,  with  gneisses  and  mica  schists  called  Her- 
cynian  in  Bavaria,  with  the  granulites  with  dichroite  rocks,  mica 
