414  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
coarse  gneisses  which  underlie  the  limestones  belong  with  the  latter 
series  or  represent  an  earlier  one,  we  have  no  knowledge.  There  is, 
then,  in  this  district  a  possible  pre-clastic  series,  which  in  its  character, 
so  far  as  seen  and  described,  is  like  the  fundamental  complex,  the  true 
Archean ;  and  a  clastic  series  of  great  but  unknown  thickness  which 
belongs  with  the  Algonkian.  That  the  Adirondack  rocks  as  a  whole 
are  unconformably  below  the  Potsdam  has  been  unquestioned  from  the 
first. 
The  relations  of  the  Highland  area  of  New  Jersey  with  the  Potsdam 
sandstone  are  such  as  to  make  it  certain  that  between  them  there  is  a 
great  structural  break.  The  rocks  comprising  this  area^consist  largely 
of  granite-gneiss,  in  general  very  nearly  massive,  but  having  a  some- 
what laminated  arrangement  of  the  mineral  constituents.  The  strike 
of  the  lamination  conforms  closely  with  the  trend  of  the  area  as  a  whole, 
being  east  of  north  and  south  of  west.  The  gneisses  over  large  areas 
are  graphitic.  Interlaminated  with  them  are  beds  of  iron  ore,  and  j  I 
apparently  of  crystalline  limestone.  They  are  cut  by  various  basic  and 
acidic  eruptives. 
The  weight  of  opinion  in  former  years  has  been  in  favor  of  the  sedi- 1 
mentary  origin  of  this  gneissic  series.  Mather,  who  gave  by  far  the 
best  early  descriptions  of  the  district,  and  Nason,  who  has  recently 
been  closely  studying  the  limestones  of  New  Jersey,  find  that  the  white 
crystalline  limestones  which  have  been  regarded  as  Archean  grade 
into  the  blue  limestones  which  are  fossiliferous.  These  writers  regard 
all  of  the  white  limestone  as  parts  of  a  newer  series  which  have  been 
metamorphosed  either  as  a  result  of  extreme  folding  or  by  intrusive 
masses  of  granite  of  later  date,  with  which  they  are  frequently  asso- 
ciated. If  all  of  these  limestones  are  excluded  from  the  pre-Cambrian '; 
and  this  is  a  very  doubtful  assumption,  the  evidence  in  favor  ,of  the 
detrital  origin  of  the  Highland  area  is  restricted  to  the  widely  dissem- 
inated graphite  and  to  the  magnetite  beds  of  iron  ore.  Magnetite  is 
widely  associated  with  certain  belts  of  the  granite-gneisses  of  New 
Jersey,  but  this  and  its  concentration  in  lenticular  masses  within  the 
gneisses  in  the  form  of  magnetite  can  hardly  be  considered  as  decisive 
evidence  of  their  sedimentary  character.  The  magnetites  associated 
with  the  basal  gabbros  of  the  lake  Superior  Keweenawan  are  in  purely 
igneous  rocks.  The  graphite  of  the  graphitic  gneiss  is  a  point  of  more 
weight.  The  absence  of  graphite  as  an  important  constituent  over 
large  areas  in  any  definitely  determined  igneous  granite- gneiss,  bears 
in  favor  of  the  sedimentary  origin  of  the  gneissic  series.  If  this  theory 
proves  true,  the  Highland  gneissic  series  more  nearly  approaches  the 
characters  of  a  massive  eruptive  than  any  other  metamorphic  sedimen- 
tary rock  known  to  the  writer.  Upon  the  whole,  in  the  regularity  of  its 
lamination,  in  its  lack  of  extreme  contortion  and  foliation,  and  in  the 
presence  of  graphite,  the  Highland  gneiss  is  not  like  the  fundamental 
complex,  the  genuine  Archean  of  Canada  and  the  West.     However, 
