476  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
GEOLOGY. 
Cumberland  Valley  is  a  broad  rolling  plain  8  miles  wide  in  the  vicinity 
of  Mount  Holly  Springs,  whence  it  extends  eastward  to  Susquehanna 
River  and  southwestward  to  Potomac  River.     It  is  limited  abruptlj 
on  the  soul  beast  by  South  Mountain,  a  tract  of  parallel  ridges  wbich 
near  Mount   Holly  Springs,  have  an  east-west  trend. 
The  valley  is  composed  largely  of  closely  folded  limestone  of  Cam] 
brian  and  Ordovician  age,  younger  Ordovician  shales  (Utica  and  Ederi 
overlying  the  limestone  along  the  northwestern  side  of  the  \  alley 
The  front  ridges  of  the  mountain  are  made  up  of  Cambrian  quartziti 
and  conglomerate  with  slate  and  soft  sandstone  valleys  between 
The  southeastern  ridges  are  of  older  volcanic  rocks,  both  rhvoliti 
and  basic. 
In  the  vicinity  of  Mount  Holly  Springs  the  general  structure  is  tha 
of  an  anticlinorium  with  the  southern  limb  covered  by  Triassic  sedi- 
ment. The  lavas  of  pre-Cambrian  age  exposed  at  the  axis  are  her 
chiefly  rhyolitic  and  have  a  marked  cleavage  or  schist osity  dippinj 
about  35°  S.,  which  obliterates  largely  the  original  banding.  Th 
basal  Cambrian  sediments  consisl  of  a  coarse  quartz  conglomerate 
massive  quartzites,  and  thin  slates.  These  are  closely  folded  an< 
overturned,  dipping  steeply  to  the  south,  although  cleavage  to  th 
s«»uth  is  so  highly  developed  that  the  stratification  is  difficult  to  deter- 
mine. In  the  longitudinal  valley  of  Mountain  Creek  a  narrow  belt  o 
overlying  limestone  is  infolded. 
Walcotl  found  fragments  of  Oh  neUus  and  Hyolithes  coin  munis  in  thd 
upper  scolithus-bearing  sandstone  jusl   above  Mount    Holly  Springs 
This  scolil  bus  bed,  which  form-  t  he  norl  h  face  of  Mount   I  lolly  Ridge] 
resembles  in  even    respeel    the  Antietam  sandstone,  the  uppermost 
sandstone  of  the  mountain-making  -eric-  of  South  Mountain  in  the! 
vicinity  of  Chambersburg,  and  the  finding  of  Georgian  (Lower  Cam! 
brian)  fossils  in  these  bed-  further  confirms  this  view,  because  fossils] 
have   not    been   found    below    this  horizon   in  South   Mountain.     The 
limestone  near  the  mountain  is  covered  by  wash.     The  nearest  out- 
crop- dip  steeply  to  the  south,  being  probably  overturned,  but  fossils 
have  not    been  found  in  them  near  Mount    Holly  Springs,  so  that  the 
exact  relation-  of  the  limestone  to  t  he  sandstone  of  t  he  mountain  have 
not  been  determined.     Walcotl   concluded"  that  a  greal  fault  exists 
along  the  west  face  of  the  mountain,  agreeing  in  this  respect  with  thel 
earlier  views  of  Lesley  and   Frazer,  as  published  in  the  reports  of  thel 
Second  Geological  Survey  of  Pennsylvania. 
From  recent  detailed  studies  in  South  Mountain  from  the  Maryland 
State  line  northward  to  the  latitude  of  Chambersburg  the  writer  has 
proved6  the  absence  of  a  fault  of  any  magnitude  along  that  part  of  the 
"  Waleott,  C.  D.,  Cambrian  rocks  of  Pennsylvania :   Bull.  I".  s.  Geol.  Survey  No.  1.34,  18%,  pp.  24-27. 
b  Jour.  Geol.,  vol.  14,  1906,  pp.  201-220. 
