614  PRE-CAMBRTAN    GEOLOGY   OF    NORTH   AMERICA. 
reference  to  one  another  that  they  seem  to  represent  remnants  of 
what  were  originally  two  continuous  parallel  northeast-southwest 
belts.  The  characteristic  rock  of  the  series  is  the  crystalline  marble. 
This  is  intricately  infolded  with  quartzose  and  hornblendic  gneisses 
and  with  fine-grained  granitic,  syenitic,  and  gabbroic  gneisses,  pre- 
cisely like  gneisses  which  occur  in  other  areas  where  no  member 
of  the  Grenville  series  is  to  be  found. 
The  gneisses  of  undetermined  age  include  granite,  syenite,  diorite, 
and  gabbro  gneisses,  together  with  intermediate  varieties.  They 
occupy  a  very  large  area.  If  all  these  gneisses  are  igneous  (as  is 
thought  probable)  there  are  three  possibilities  in  regard  to  their  age: 
1.  They  may  represent  in  whole  or  in  part  a  more  ancient  series 
than  the  Grenville. 
2.  They  may  represent  a  somewhat  later  series  intrusive  in  the 
Gremulle,  but  older  than  the  great  gabbro,  syenite,  and  granite 
intrusions. 
3.  They  may  represent  thoroughly  foliated  phases  of  these  later 
intrusions. 
In  Cushing's  present  judgment  they  will  be  found  to  belong  partly 
under  2  and  partly  under  3,  but  more  especially  the  former. 
No  rocks  have  been  found  in  the  northern  Adirondacks  which  can 
be  shown  to  be  older  than  the  Grenville  series,  but  in  every  case  in 
which  the  relations  have  been  made  out  the  adjacent  rocks  show 
intrusive  contacts  with  the  Grenville  rocks.'  On  the  other  hand,  the 
Grenville  is  a  sedimentary  series  and  must  have  been  laid  down  on 
some  floor. 
Younger  than  the  Grenville  rocks  and  for  the  most  part  younger 
than  the  doubtful  gneisses  are  a  considerable  quantity  of  igneous 
rocks,  comprising  gabbros  (anorthosites),  syenites,  and  granites. 
These  again  occupy  large  areas. 
In  the  northern  portion  of  the  county  Upper  Cambrian  rocks  over- 
lie the  pre-Cambrian  rocks  with  unconformity. 
Smyth,53  in  1901,  describes  and  maps  the  geology  of  the  crystal- 
line rocks  in  the  vicinity  of  St.  Lawrence  River  in  the  town  of  Alex- 
andria and  parts  of  Clayton  and  Theresa  in  Jefferson  County,  to- 
gether with  portions  of  Rossie  and  Hammond  in  St.  Lawrence 
County.  Within  this  area  are  included  the  Thousand  Islands  of  St. 
Lawrence  River.  The  four  rock  types  are  crystalline  limestone  with 
schists,  gneiss,  granite,  and  Potsdam  sandstone.  The  term  schists  is 
here  used  to  include  a  variety  of  rocks,  such  as  quartzite,  hornblende 
and  mica  schists,  hornblende,  pyroxene,  and  mica  gneisses,  etc.  The 
crystalline  limestones  and  associated  schists  have  for  the  most  part 
the  aspect  of  sedimentary  rocks  and  are  so  classed.  The  gneiss  is  a 
complex  so  far  as  age  is  concerned,  although  the  differences  in  the 
ages  of  the  members  are  slight  and  discrimination  of  the  units  is  not 
