666  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
were  deposited.  This  hypothesis  accounts  for  the  difference  in  crys- 
talline character  between  the  rocks  of  the  two  areas,  for  the  abrup. 
ness  of  their  contact,  and,  since  both  series  have  been  subjected  to  a 
folding  together,  for  their  apparent  conformity  along  their  contact. 
As  to  the  age  of  the  rocks,  it  is  probable  that  the  Paleozoic  should  in- 
clude all  the  semicrystalline  schists,  while  the  noncrystalline  rocks 
east  of  them  would  be  assigned  to  the  Algonkian  or  Archean. 
Keyes,27  in  1891,  gives  as  a  supplement  to  the  preceding  a  section 
across  the  Piedmont  Plateau  of  Maryland.  In  the  Frederick  lime- 
stone of  the  western  semicrystalline  rocks  are  fossils  of  several  types 
characteristic  of  the  Trenton,  and  the  entire  series  of  limestones  and 
shales  probably  represent  the  Chazy,  Trenton,  and  Hudson  River 
formations.  East  of  the  western  semicrystalline  rocks  are  contorted 
gneisses,  with  general  westerly  dips,  which  are  cut  by  basic  and  acidic 
rocks  and  which  are  believed  to  have  been  originally  granitic,  but 
through  the  agency  of  enormous  orographic  pressure  have  been 
squeezed  into  their  present  gneissic  condition,  as  shown  by  the  me- 
chanical deformations  through  which  the  grains  have  gone. 
Keith,28  in  1892,  summarizes  the  geologic  structure  of  the  Blue 
Ridge  in  Maryland  and  Virginia.  Considered  in  a  general  way,  the 
region  is  a  broad  anticline  and  successively  younger  rocks  appear  east 
and  west  from  the  nucleus  of  igneous  rocks.  This  arch  is  crumpled 
into  several  synclines  and  broken  by  faults  that  follow  the  mountain 
lines  rather  closely.  The  igneous  rocks  near  the  nucleus  are  the  only 
pre-Cambrian  rocks  present.  The  physical  history  of  the  area  is,  in 
brief : 
Surface  flow  of  quartz  porphyry,  followed  after  short  interval  by  surface 
flow  of  diabase.  Injection  of  granite  into  the  diabase,  presumably  passing 
through  the  quartz  porphyry.  Dynamic  action  and  production  of  schistosity 
in  diabase. 
Erosion. 
Submergence  and  deposition  of  Paleozoic  strata.  Dynamic  action  with  fold- 
ing, cleavage,  and  elevation. 
Erosion. 
Submergence  and  Juratrias  deposition. 
Williams,29  in  1892,  describes  the  volcanic  rocks  of  South  Mountain 
and  emphasizes  their  volcanic  character.  Most  of  these,  with  the 
exception  of  a  few  of  the  most  massive  greenstones,  have  hitherto 
been  regarded  as  of  sedimentary  origin,  because  of  their  accompany- 
ing accumulation  of  tuff  beds  and  breccias,  and  the  fact  that  they  are 
generally  cleaved  parallel  to  the  great  structure  planes  of  the  moun- 
tain.    The  rocks  are  principally  rhyolites  and  basalts. 
Walcott,30  in  1892,  determines  the  Cambrian  age  of  the  quartzite 
bordering  on  the  igneous  rocks  of  South  Mountain  and  its  southern 
extension,  the  Blue  Ridge.  The  underlying  igneous  rocks  are  there- 
fore pre-Cambrian. 
