90  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
forming  the  lower  members  of  the  group  in  the  Primitive  terrane, 
while  the  argillites,  found  toward  the  summit  of  the  same  group,  were 
regarded  as  constituting  the  lower  division  of  the  Transition  terrane, 
covered,  according  to  him,  unconformably  by  the  fossiliferous  gray- 
wacke  (first  graywacke)  which  formed  the  upper  division  of  the  same 
Transition  terrane.  Emmons,  on  his  part,  in  1842,  comprised  in  what 
he  called  the  Taconic  sj^stem  all  this  crystalline  series,  as  well  as  the 
graywacke ;  but  in  1844  he  separated  the  latter,  in  which  he  had  recog- 
nized the  existence  of  a  trilobitic  fauna,  giving  it  the  name  Upper 
Taconic.  Long  studies  have  convinced  the  senior  author  that  this 
upper  division  is  entirely  independent  of  the  Lower  Taconic,  with 
which  this  fossiliferous  graywacke  is  in  contact  only  in  relatively 
restricted  regions,  while  in  other  localities  it  rests  directly  on  older 
crystalline  terranes.  Seeing,  moreover,  that  the  Lower  Taconic  is 
found  alone  in  a  great  number  of  localities  from  the  Gulf  of  St.  Law- 
rence to  Alabama  on  the  south  and  to  Lake  Superior  on  the  west,  and 
recognizing  also  the  fact  that  the  Upper  Taconic  really  forms  part  of 
the  Cambrian  terrane  (as  was  recognized  by  Emmons  in  1860),  the 
author  proposed  as  far  back  as  1878  to  restrict  the  employment  of  the 
term  Taconic  to  this  crystalline  and  infra-Cambrian  series  which 
forms  the  Lower  Taconic  of  Emmons  and  the  Itacolumitic  group  of 
Lieber,  and  to  give  it  the  name  Taconian  terrane. 
The  mineralogical  resemblances  existing  among  the  various  crystal- 
line terranes  mentioned  above  are  easy  to  recognize.  The  type  of 
rocks  with  orthose  base  that  appears  in  the  fundamental  granite  and 
the  Laurentian  gneisses  is  also  found  in  the  quartziferous  porphyries 
of  the  Arvonian  and  the  gneiss  of  the  Montalban,  and  less  distinctly 
in  the  feldspathic  rocks  of  the  Taconian.  The  nonmagnesian  micas, 
rare  in  the  fundamental  granite  and  the  Laurentian  gneisses,  are 
found  abundantly  represented  in  the  gneisses  and  mica  schists  of 
the  Montalban,  as  well  as  in  the  lustrous  schists  found  in  the  Hu- 
ronian  and  Taconian  terranes,  and  even  predominate  in  the  latter. 
It  is  still  to  be  remarked  that  the  simple  silicates  of  alumina,  such  as 
andalusite,  disthene,  fibrolite,  and  pyrophyllite,  which  seem  foreign 
to  the  oldest  terranes,  abound  in  the  Montalban  and  also  appear  in 
the  Taconian.  At  the  same  time  the  crystalline  limestones,  the  oxides 
of  iron,  and  the  calcareous  and  magnesian  silicates  are  found  repre- 
sented in  each  terrane  beginning  from  the  fundamental  granite.  The 
chemical  and  mineralogical  differences  between  these  various  terranes 
are  greater  than  the  resemblances,  which  has  not  prevented  certain 
observers  from  confounding  the  recent  gneiss  with  the  old  gneiss. 
In  fact,  the  resemblances  between  the  Huronian  and  Taconian  ter- 
ranes have  led  the  late  Professor  Kerr,  in  North  Carolina,  to  refer 
the  latter  terrane  to  the  Huronian.  In  the  vicinity  of  Lakes  Su- 
perior and  Huron,  too,  where  the  Laurentian,  Norian,  Arvonian, 
