PIEDMONT  PLATEAU  AND  PORTIONS  OF  THE  APPALACHIANS.       671 
sedimentary  rocks  of  known  horizon.  On  the  east  Baseom  has  con- 
sidered that  the  Baltimore  gneiss  is  pre-Cambrian,  the  quartzite 
Cambrian,  the  limestone  Cambro-Ordovician,  and  the  Wissahickon 
Ordovician.  On  the  west  the  phyllites  are  apparently  in  the  same 
relation  to  the  known  Cambrian  and  Cambro-Silnrian  deposits,  al- 
though in  this  area  the  geological  mapping  has  not  been  conducted 
in  detail  on  account  of  the  lack  of  satisfactory  topographic  maps. 
The  conclusion  from  the  facts  at  hand  Avould  therefore  seem  to  war- 
rant the  following  correlation : 
Correlation  of  Piedmont  of  Maryland  and  Pennsylvania. 
Maryland  Piedmont. 
Western. 
Central. 
Eastern. 
Pennsylvania  Piedmont. 
Southern. 
Eastern. 
Ordovician 
Cambro-Ordo- 
vician. 
Cambrian 
Pre-Cambrian 
[Ma  ssanutten 
sandstone. 
Martinsburg 
[    shale. 
Shenandoah 
limestone. 
Antietam  sand- 
stone. 
Harpers  shale  . . 
We  verton  sand- 
stone. 
Loudon  iorma- 
tion. 
Granites, 
schists,  etc. 
Phyllite       and 
Wissahickon 
schists. 
Cockeysville 
marble. 
Setters   quart z- 
I     ite  with  argil- 
laceous upper 
member. 
Baltimore 
gneiss. 
Peachbottom 
slate. 
Cardiff  quartz- 
ite. 
Wissahickon 
formation. 
Baltimore 
gneiss. 
Wissali  ickon 
formation. 
Chester  Valley. 
/C  h  i  c  k  i  e 
(    quartzite. 
Baltimore 
gneiss. 
Hudson  River. 
Cambro-  Silu- 
rian. 
Cambrian. 
Pre-Cambrian. 
Opposed  to  this  conclusion  is  the  fact  that  directly  on  the  strike  of 
the  Wissahickon  schist  is  a  series  of  more  quartzose  and  compact 
metamorphosed  sediments  wmich  have  been  regarded  by  Keith  in  his 
discussion  of  the  geology  of  the  Washington  quadrangle  as  Carolina 
gneiss,  which,  as  defined  by  him,  is  pre-Cambrian.  The  grounds  for 
assigning  such  an  age  to  rocks  of  this  region  involve  the  areal  work 
of  Keith  in  the  region  to  the  north  and  west  of  Washington,  which 
has  not  yet  been  published  on  account  of  the  lack  of  a  proper  topo- 
graphic base. 
The  Baltimore  gneiss,  the  onl}7  pre-Cambrian  formation,  occurs  in 
several  well-defined  areas  in  the  eastern  portion  of  the  Piedmont  be- 
tween Susquehanna  and  Potomac  rivers.  The  easternmost  of  these 
Baltimore  gneiss  occurrences  is  within  the  area  of  Cecil  County,  easl 
of  Susquehanna  River,  studied  by  Baseom,  and  extends  from  this 
point  southward,  widening  to  an  area  of  5  miles  or  more  in  breadth 
where  it  is  overlain  by  Coastal  Plain  deposits.  This  formation  is 
limited  on  either  side  by  igneous  rocks.  An  outlier,  a  mile  or  l<*>s  in 
width,  extending  for  several  miles  southward  from  Susquehanna 
River  probably  represents  a  detached  portion  of  the  larger  mass  lying 
a  little  to  the  south. 
