412  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull.  86. 
LITERATURE   OF   DELAWARE. 
Booth,170  in  1841,  includes  among  the  primary  rocks  gneiss,  feld- 
spathic  rocks,  limestones,  serpentine,  and  granite,  the  first  comprising 
about  three-quarters  of  the  area.  This  region  is,  without  question, 
stratified.  The  average  bearing  of  the  rocks  is  north  47°  east,  and  the 
dip  70°  northwest,  but  occasional  bearings  are  found  which  differ  widely 
from  this,  and  the  dip  is  vertical.  The  trap  rocks  have  a  dip  and 
strike  conformable  with  the  gneiss  and  grade  into  them.  The  lime- 
stone is  a  coarse  to  fine  grained  crystalline  marble,  interstratified  with 
the  gneiss.  The  serpentine  and  surrounding  rocks  are  cut  by  numer- 
ous veins  of  granite*  The  greater  part  of  the  trappean  formation  pos- 
sesses a  clearly  stratified  structure  and  grades  by  transition  into  the 
gneiss,  but  the  hornblendic  and  coarse  feldspathic  veins  do  not.  The 
variation  in  the  strike  and  dip  of  the  gneiss  is  regarded  as  due  to  the; 
granitic  veins  or  to  the  serpentine. 
Chester,171  in  1885,  places  in  the  Laurentian  the  hornblendic  rocks] 
along  the  line  of  the  Pennsylvania  railroad  and  the  area  to  the  east  oi 
West  Chester.  The  rock  is  a  dark  hornblendic  gneiss  or  amphibolitid 
schist,  with  which  is  associated  a  dioritic  or  syenitic  granite  of  the 
Pennsylvania  survey.  The  two  rocks  grade  into  each  other,  and  prob-j 
ably  form  varieties  of  the  same  eruptive  series.  North  of  the  Lauren] 
tian  gneisses,  and  resting  upon  them,  is  a  series  of  mica- schists  and 
granitic  gneisses,  with  which  are  associated  bedded  granites,  serpen-) 
tines,  and  hornblende  rocks  which  have  been  referred  to  the  Montalj 
ban,  or,  with  the  Laurentian,  have  been  called  Azoic.  These  do  noq 
form  two  successive  formations,  for,  while  the  former  is  either  Lauren-] 
tian  or  Huronian,  the  latter  must  be  placed  above  the  Trenton,  and 
possibly  above  the  Hudson  river  slates.  The  granite  of  the  State  is  id 
intrusive  beds  and  in  beds  which  are  no  more  than  highly  metamor-l 
phosed  granitic  gneiss  or  mica-schist,  the  two  latter  grading  into  each] 
other.  Crystalline  limestones  are  found  at  Pleasant  hill,  HockessinJ 
and  near  Centreville.  Serpentine  is  found  northeast  of  Wilmington  as  a 
dike,  running  with  the  micaceous  schist.  Vitreous  quartz  and  quartzitd 
occur  as  thin  or  massive  seams  interstratified  with  the  micaceous  rocks.1 
The  quartzite  of  the  northeast  corner  of  the  State,  underlying  limestone,] 
is  probably  of  Potsdam  age.  The  strikes  and  dips  of  the  crystalling 
rocks  are  very  variable,  and  this  variation  is  often  due  to  the  disturb-l 
ing  action  of  granitic  intrusions.  The  Laurentian  is  an  extension  of 
the  third  belt  of  Rogers.  The  limestones  are  younger  than  the  PotsJ 
dam  quartzites,  and  are  regarded  as  calciferous;  the  mica-schists  and 
gneisses  certainly  overlie  the  limestones,  and  the  latter  therefore  begin] 
somewhere  in  the  Silurian,  and  possibly  mount  as  high  as  the  Devo-J 
nian. 
Chester,172  in  1890,  describes  the  gabbros,  gabbro-diorites,  and  horn-] 
blende-schists  of  Delaware  and  their  relations  to  the  surrounding  rocks.1 
