van  rise.J  EASTERN    UNITED    STATES.  413 
The  gabbro,  gabbro- diorites,  and  hornblende-schists  are  found  to  grade 
into  each  other  by  imperceptible  stages,  and  the  two  latter  are  regarded 
as  a  metamorphosed  product  of  the  former  and  all  of  igneous  origin. 
These  rocks  are  found  at  various  points  in  contact  with  the  mica-schists 
and  gneisses.  Where  the  eruptive  rocks  have  a  schistose  structure 
this  is  in  apparent  conformity  with  the  foliation  of  the  mica-schists. 
Sometimes  the  mica-schists  appear  to  dip  beneath  the  eruptive  rocks, 
and  at  other  times  to  overlie  them.  No  evidence  was  found  of  any 
bedding  not  coincident  with  the  cleavage.  The  unconformity  discov- 
ered by  Hall  between  the  trappean  rocks  and  mica-schists  in  Dela- 
ware county,  Pennsylvania,  was  not  found  on  the  Delaware  side  of 
the  line.  If  the  horizontal  bedding  described  exists,  it  is  so  obscured 
as  to  be  unrecognizable;  if  the  mica-schists  are  considered  to  lie  hori- 
zontally the  eruptive  character  of  the  gabbros,  gabro-diorites,  and 
hornblende  rocks  is  but  the  more  evident. 
GENERAL  LITERATURE. 
Martin,173  in  1886,  states  that  the  Tide-water  gneiss  has  mineralogi- 
cal  characters  which  distinguishes  it  strongly  from  the  gneisses  of  the 
northern  Laurentian  and  from  the  Highlands.  In  particular  the 
abundance  of  subsilicates  and  of  hydrous  silicates  is  to  be  noted. 
SUMMARY   OF  RESULTS. 
The  literature  summarized  clearly  shows  that  in  the  central  Appala- 
chian area,  as  in  the  northern,  only  general  certain  results  have  been 
reached.  While  there  are  numerous  areas  of  crystalline  rocks,  it  is  not 
clear  that  many  are  of  pre- Cambrian  age.  The  exceptions  are  the  Adi- 
rondacks ;  the  Highlands  of  New  Jersey,  with  its  norther^  extension 
the  Highlands  of  New  York,  and  its  southern  extension  South  moun- 
tain of  Pennsylvania;  and  the  eastern  area  of  Maryland.  A  part  of 
the  northern  extension  of  the  Blue  ridge  is  probably  also  to  be  here 
included. 
The  most  widespread  rock  in  the  Adirondacks  is  a  gabbro,  which  has 
all  of  the  characteristics  of  this  eruptive  rock.  The  outer  border  of  the 
mass  has  a  well  laminated  structure,  due  either  to  original  crystalliza- 
tion or  to  subsequent  metamorphism.  This  rock  is  in  all  respects  like 
the  Labradorian  or  Norian  of  the  Canadian  survey,  to  which  indeed  it 
has  been  referred  by  the  advocates  of  the  Norian  system.  The  bedded 
succession  of  gneisses,  limestones,  and  quartzites  is  in  nearly  all  re- 
spects like  the  original  Laurentian  described  by  Logan.  While  now  it 
nowhere  has  indubitable  clastic  characters,  its  beds  are  such  as  to  show 
that  it  was  originally  a  sedimentary  series.  Since  the  series  is  not 
closely  folded,  probably  the  principal  cause  of  its  metainorphism  is  the 
great  batholitic  mass  of  gabbro  occupying  the  core  of  the  mountains, 
which  seems  to  have  thrust  itself  up  among  the  elastics.     Whether  the 
