660  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
ilar  series  of  formations  are  known  to  be  of  the  same  age.  What  is 
happening  to-day  has  happened  in  all  ages.  Nothing  could  be  more 
unlike  than  the  deposits  now  forming  along  the  various  ocean  shores 
and  in  different  lakes  and  inland  seas,  yet  they  are  all  of  one  age. 
Even  the  deposits  making  in  one  and  the  same  basin  radically  differ ; 
as,  for  example,  along  the  northern  and  southern  sides  of  Lake  On- 
tario and  along  the  eastern  and  western  sides  of  Lake  Champlain. 
It  would  therefore  seem  a  useless  task  to  seek  for  the  Huronian  rocks 
far  from  their  native  range. 
Bascom,20  in  1896,  describes  and  maps  pre-Cambrian  volcanic  rocks 
of  South  Mountain,  Pennsylvania.  The  volcanic  rocks  are  both  basic 
and  acidic.  The  acidic  rocks  comprise  quartz  porphyries,  devitri- 
fied  rhyolites  or  aporhyolites,  with  accompanying  pyroclastics,  and 
sericite  schists,  the  last  being  the  metamorphosed  forms  of  the  quartz 
porphyries  and  aporhyolites.  The  basic  rocks  comprise  melaphyres, 
augite  porphyrites,  slates,  and  pyroclastics.  Lithologically  the  vol- 
canic rocks  resemble  the  Keweenawan  copper-bearing  rocks  of  Lake 
Superior. 
There  is  not  sufficient  evidence  to  decide  the  comparative  age  of 
the  basic  and  acidic  rocks,  but  field  observations  in  the  Monterey  dis- 
trict indicate  that  the  acidic  rocks  are  the  older.  The  volcanics  are 
overlain,  with  stratigraphical  unconformity  but  with  structural  con- 
formity, by  sedimentary  rocks  of  Lower  Cambrian  age.  Both  vol- 
canics and  sedimentaries  have  been  subjected  to  strong  dynamic 
action,  whereby  the  igneous  rocks  have  been  cleaved  and  sheared  and 
the  sedimentary  rocks  thrust  over  them  from  the  east. 
Bascom,21  in  1905,  describes  the  geology  of  the  Piedmont  district 
of  Pennsylvania.  The  summary  on  the  following  pages  by  Doctor 
Bascom  practically  covers  the  same  ground  and  will  not  here  be 
repeated. 
See  also  summaries  of  work  of  Keith,  Walcott,  and  others  under 
Maryland,  section  2. 
SUMMARY    OF    PRESENT    KNOWLEDGE. 
EASTERN    PIEDMONT    BELT.a 
The  Piedmont  district  of  Pennsylvania,  forming,  with  a  width  of 
65  miles,  the  southeast  border  of  the  State,  constitutes  an  important 
part  of  the  Atlantic  Piedmont,  and  is  in  geology  and  physiography 
an  epitome  of  the  larger  district.  Geologically  it  is  a  complex  of 
highly  metamorphosed  sedimentary  and  intrusive  igneous  materials 
largely  concealed  beneath  a  cover  of  Mesozoic  sandstone  and  shale 
and  of  Cenozoic  gravel,  clay,  marl,  and  sand.     This  cover  conceals 
0  The  summary  for  the  eastern  Piedmont  belt  has  been  prepared  for  this  bulletin  by 
Dr.  Florence  Bascom. 
