2G8  PRE-CAMBRTAN    ROCKS    OF    NORTH    AMERICA.  (bull. 86. 
of  any  other  pre-Cambrian  system.  While  in  the  main  there  is  little 
difficulty  in  distinguishing  between  the  Fernandan  and  Burnetan  strata, 
nietamorphism  has  caused  a  close  resemblance  in  many  exposures. 
The  general  succession  from  above  downward  is  calcareous  rock,  chlo- 
ritic  slates  and  shales,  carbonaceous  schists,  ferruginous  rocks,  quartz- 
ites,  acidic  schists,  and  basic  schists.  In  this  system  are  various  erup- 
tive*, including  granites,  quartz  dikes,  and  basic  rocks.  Whenever  the 
Fernandan  beds  are  visible  in  connection  with  the  Burnetan  strata, 
through  their  own  excessive  erosion  or  by  reason  of  the  persistence  of 
prior  elevations  of  the  earlier  system,  there  is  always  abundant  evidence 
of  unconformity ;  and  if  any  fractures  occur  the  joints  of  the  northwest 
(Fernandan)  trend  invariably  cross  and  cut  the  strike  of  the  Burnetan 
rocks.  Additional  support  for  the  unconformity  of  the  two  systems  is 
gained  from  the  fact  that  contortions  occur  in  the  lower  system  only 
where  this  or  later  trends  affect  its  continuity.  Moreover,  the  compo- 
sition and  texture  of  the  Fernandan  beds  are  to  a  large  extent  that  of 
derivatives  of  the  Burnetan  lithologic  series. 
Above  the  Fernandan  system  is  an  Eparchean  group  of  rocks,  the 
stratigraphical  affinities  of  which  are  nearer  the  Archean  than  the 
Cambrian.  There  is  no  doubt  that  they  rest  unconformably  below  the 
Paleozoic.  To  this  group,  including  Walcott's  Llano  group,  is  given 
the  term  Texan  (Algonkian^)  system.  The  rocks  of  the  Texan  system 
are  chiefly  siliceous,  but  shales  and  limestones  are  not  wanting.  The 
succession  includes  from  the  base  upward  a  set  of  micaceous  sand- 
stones, with  thinly  laminated  shales  and  chloritic  detrital  material; 
hard,  white  laminated  quartz  rock  or  quartzite,  associated  with  fer- 
ruginous and  schists  layers;  ferruginous  shale  beds,  in  part  somewhat 
graphitic,  and  limestones  or  marbles.  It  is  often  difficult  in  the  field 
to  distinguish  the  graphitic  shale  and  marble,  as  a  belt,  from  the  simi- 
lar lithologic  set  of  the  earlier  Fernandan  system.  In  hand  specimens, 
however,  the  distinction  is  obvious.  The  Texan  beds  are  much  less 
altered,  as  a  rule.  The  graphitic  strata  are  plainly  derivatives  of  the 
preexisting  graphite  schists,  and  the  marbles  are  white  or  brown,  in- 
stead of  blue.  The  Packsaddle  marbles  and  shaly  beds  are  compared 
with  the  Ohuar;  the  Llano  quartzites  and  sandstones,  with  eruptives, 
to  the  Grand  canyon,  and  the  Mason  sandy  shales  and  schists  to  the] 
Vishnu  series. 
There  must  have  been  a  vast  amount  of  erosion  after  the  folding  of 
the  Texan  strata  and  prior  to  the  deposition  of  the  Cambrian  sedi- 
ments upon  the  upturned  edges.  The  outcrops  of  the  Texan  strata  are 
almost  invariably  accompanied  by  some  of  the  Fernandan  beds,  or  byJ 
members  very  closely  resembling  these,  often  in  such  relations  as  to'] 
make  it  difficult  to  determine  the  boundary  between  the  two  groups  , 
upon  structural  grounds  alone;  but  the  rocks  here  included  as  of  the  j 
Texan  system  are  never  involved  in  an  earlier  uplift  than  the  north-] 
south  trend. 
