676  PRE-CAMBRIAN   GEOLOGY   OF   NORTH   AMERICA. 
SECTION  4.     DELAWARE. 
SUMMARY   OF  LITERATURE. 
Booth,59  in  1841,  includes  among  the  Primary  rocks  gneiss,  feld- 
spathic  rocks,  limestones,  serpentine,  and  granite,  the  first  compris- 
ing about  three-quarters  of  the  area.  This  region  is,  without  ques- 
tion, stratified.  The  average  bearing  of  the  rocks  is  N.  47°  E.,  and 
the  average  dip  70°  NW.,  but  occasionally  bearings  are  found  which 
differ  widely  from  this,  as  well  as  vertical  dips.  The  trap  rocks  are 
in  dip  and  strike  comformable  with  the  gneiss  and  grade  into  it.  The 
limestone  is  a  coarse  to  fine  grained  crystalline  marble,  interstratified 
with  the  gneiss.  The  serpentine  and  surrounding  rocks  are  cut 
by  numerous  veins  of  granite.  The  greater  part  of  the  trappean 
formation  possesses  a  clearly  stratified  structure  and  grades  by  tran- 
sition 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,00  in  1885,  places  in  the  Laurentian  the  hornblendic  rocks 
along  the  line  of  the  Pennsylvania  Railroad  and  the  area  to  the  east 
of  West  Chester.  The  rock  is  a  dark  hornblendic  gneiss  or  amphib- 
olitic  schist,  with  which  is  associated  a  dioritic  or  syenitic  granite 
of  the  Penns}dvania  Survey.  The  two  rocks  grade  into  each  other, 
and  probably  form  varieties  of  the  same  eruptive  series.  North  of 
the  Laurentian  gneisses,  and  resting  upon  them,  is  a  series  of  mica 
schists  and  granitic  gneisses,  with  which  are  associated  bedded  gran- 
ites, serpentines,  and  hornblende  rocks  which  have  been  referred  to 
the  Monta'lban,  or,  with  the  Laurentian,  have  been  called  Azoic. 
These  do  not  form  two  successive  formations,  for,  while  the  former 
is  either  Laurentian  or  Huronian,  the  latter  must  be  placed  above  the 
Trenton,  and  possibly  above  the  Hudson  River  slates.  The  granite 
of  the  State  is  in  intrusive  beds  and  in  beds  which  are  no  more  than 
highly  metamorphosed  granitic  gneiss  or  mica  schist,  the  two  latter 
grading  into  each  other.  Crystalline  limestones  are  found  at  Pleas- 
ant Hill,  Hockessin,  and  near  Centerville.  Serpentine  is  found 
northeast  of  Wilmington  .as  a  dike,  running  with  the  micaceous 
schist.  Vitreous  quartz  and  quartzite  occur  as  thin  or  massive  seams 
interstratified  with  the  micaceous  rocks.  The  quartzite  of  the  north- 
east corner  of  the  State,  underlying  limestone,  is  probably  of  Pots- 
dam age.  The  strikes  and  dips  of  the  crystalline  rocks  are  very  vari- 
able, and  this  variation  is  often  due  to  the  disturbing  action  of  gran- 
itic intrusions.  The  Laurentian  is  an  extension  of  the  third  belt 
of  Rogers.  The  limestones  are  younger  than  the  Potsdam  quart- 
zites,  and  are  regarded  as  Calciferous;  the  mica  schists  and  gneisses 
certainly  overlie  the  limestones,  and  the  latter  therefore  begin  some- 
where in  the  Silurian,  and  possibly  mount  as  high  as  the  Devonian. 
