704  PRE-CAMBRIAS    GEOLOGY   OF    NORTH   AMERICA. 
and  Flattop  schist,  in  countless  small  beds,  and  it  forms  larger 
masses  as  a  surface  flow.  The  formation  consists  of  dark-blue  or 
gray  metarhyolite  in  which  flow  banding  and  phenocrysts  are  common 
and  lithophystc  and  amygdules  are  often  to  be  found.  The  pheno- 
crysts consist  of  quartz  and  feldspar,  and  the  groundmass  is  of  the 
same  minerals  with  a  very  fine  grain.  Much  of  the  formation  is 
metamorphosed  to  dark  schists  composed  chiefly  of  fine  quartz  and 
muscovite.  Large  bodies  of  this  formation  are  found  in  Maryland 
and  southern  Pennsylvania  and  have  been  described  under  the  above 
heading  and  as  aporhyolite.  They  are  substantially  the  same  as  the 
metarhyolite  described  in  North  Carolina  and  occurring  elsewhere 
in  South  Carolina,  North  Carolina,  and  eastern  Virginia. 
Correlation. — The  enormous  extent  of  the  Paleozoic  formations  is 
well  known.  Single  formations — for  instance,  the  great  Cambro- 
Ordovician  limestone — reach  continuously  from  Pennsylvania  to 
Alabama.  Recent  investigations  have  shown  an  equally  great  extent 
for  many  of  the  igneous  rocks.  For  example,  the  Roan  gneiss — an 
altered  basic  igneous  rock — extends  with  almost  perfect  continuity 
from  northern  Georgia  nearly  to  Maryland.  Masses  of  granite  have 
as  great  a  length.  The  Carolina  gneiss  has  an  even  greater  length 
and  a  breadth  in  North  Carolina  of •  more  than  100  miles.  The  rocks 
of  the  soapstone  group  appear  at  short  intervals  with  the  same 
characteristics  throughout  the  entire  extent  of  the  Appalachians. 
The  pre-Cambrian  lavas  extend  from  southern  Pennsylvania  half- 
way through  Virginia  without  a  break,  and  farther  south  they  form 
many  areas  30  or  40  miles  in  length.  In  short,  since  practically  every 
formation  has  a  great  linear  extent,  the  conditions  which  produced 
the  formations  are  of  equal  or  greater  extent. 
The  determinations  of  age  for  the  various  pre-Cambrian  forma- 
tions rest  upon  a  very  large  basis  of  fact.  The  oldest  rocks  the  age 
of  which  is  determined  by  fossils  are  the  lower  Cambrian  quartzites, 
outcropping  along  the  northwestern  front  of  the  Appalachian  Moun- 
tains. These  are  overlain  by  various  calcareous  formations  of  the 
Cambrian  and  Ordovician,  which  also  contain  fossils.  At  hundreds 
of  localities  between  Pennsylvania  and  Georgia  fragments  of  the 
igneous  rocks  are  found  in  the  Cambrian  strata.  In  the  Cambrian 
basal  conglomerates  nearly  all  of  the  igneous  and  metamorphic  rocks 
are  represented,  but  most  of  the  fragments  are  the  harder  materials, 
including  granite,  quartz,  and  the  jasper,  epidote,  and  metarhyolite 
from  the  lavas.  The  fragments  vary  from  arkose  to  rounded  pebbles 
or  to  bowlders  a  foot  or  two  in  diameter.  The  interval  of  erosion 
between  the  Cambrian  and  pre-Cambrian  rocks  proved  by  these 
conglomerates  might  be  long  or  short,  so  far  as  the  sediments  show. 
The  internal  evidence  of  the  metamorphic  and  igneous  rocks,  how- 
ever, is  abundant. 
