vanhise.]  EASTEKN    UNITED    STATES.  415 
there  are  no  certain  criteria  upon  which  it  can  be  referred  either  to 
the  Algonkian  or  Archean.  It  must  be  simply  classified,  so  far  as  pres- 
ent knowledge  goes,  as  pre-Cambrian. 
If  it  can  not  yet  be  decided  whether  the  Highland  gneisses  are  sedi- 
mentary the  supposed  structural  divisions  of  Britton  and  Nason  can 
be  regarded  as  only  lithological.  Britton's  arrangement  of  a  massive 
group  in  the  cores  and  schistose  groups  on  the  outer  parts  of  the  ranges 
can  be  as  well  explained,  as  has  been  repeatedly  seen,  by  the  eruptive 
theory  of  the  origin  of  the  series  as  by  the  sedimentary.  From  Mason's 
work  it  appears  that  certain  varieties  of  rock  have  a  continuous  wide- 
spread distribution ;  but  the  descriptions  show  that  his  various  types 
grade  into  each  other  instead  of  being  sharply  differentiated  as  supposed. 
Magnetite  is  the  distinguishing  characteristic  of  one  type,  and  yet,  in 
order  to  make  out  the  continuity  of  this  belt,  rocks  have  to  be  classed 
with  this  type,  in  which  hornblende  and  biotite  are  the  chief  basic  con- 
stituents. The  same  thing  is  true  of  the  second  type,  in  which  the  horn- 
blende, the  distinguishing  characteristic,  is  locally  almost  wholly  re- 
placed by  magnetite  or  biotite. 
Of  the  eastern  Crystalline  area  of  Maryland  nothing  can  be  said  as 
to  age,  except  that  it  is  pre-Cambrian. 
The  work  of  Mather  and  Dana  in  eastern  New  York  and  on  Man. 
hattan  island,  the  work  of  Emerson,  Dale,  Wolff,  and  Pump.elly  in  the 
adjacent  district  in  Massachusetts,  combined  with  the  paleontological 
work  of  Walcott,  show  beyond  all  reasonable  doubt  that  a  considera- 
ble part  of  the  crystalline  area  of  southeastern  New  York,  including  in 
all  probability  Manhattan  island  itself  and  the  so-called  Taconics,  be- 
long with  the  Cambrian  and  post-Cambrian  formations.  The  detailed 
evidence  for  this  is  rather  for  another  to  consider. 
Various  other  crystalline  areas  in  southeastern  Pennsylvania,  in 
Maryland  and  in  Delaware,  are  in  large  measure  metamorphosed  Cam- 
brian and  post-Cambrian  rocks,  as  shown  by  the  work  of  Bogers,  Hall, 
Williams,  and  Chester.  There  are  also  probably  in  these  areas  pre- 
Cambrian  rocks,  although  often  the  gradations  described  between  the 
gneissic  series,  supposed  to  be  pre-Cambrian  and  the  crystalline 
schists  supposed  to  be  Cambrian  or  post-Cambrian,  are  so  complete  as 
to  leave  the  reader  quite  in  doubt  as .  to  the  reality  of  the  break  sup- 
posed to  exist  between  them. 
Among  all  the  earlier  writers  on  the  crystalline  rocks  of  the  Middle 
Atlantic  states,  Mather  is  distinguished  for  the  fidelity  of  his  descrip- 
tions and  for  the  keenness  of  his  insight.  While  in  his  great  New 
York  report  of  1843  there  are  some  crude  notions,  the  comprehensive 
general  results  announced  accord  to  a  remarkable  degree  with  the 
views  held  by  the  best  informed  of  the  geologists  who  are  working  in 
this  field  to-day. 
