ISOLATED   AREAS   TN    MISSISSIPPI   VALLEY.  745 
chloritic  slates  and  shales,  carbonaceous  schists,  ferruginous  rocks, 
quartzites,  acidic  schists,  and  basic  schists.  In  this  system  are  vari- 
ous eruptives,  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  oc- 
cur, 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  con- 
tortions occur  in  the  lower  system  only  where  this  or  later  trends 
affect  its  continuity.  Moreover,  the  composition  and  texture  of  the 
Fernandan  beds  are  to  a  large  extent  that  of  derivatives  of  the 
Burnetan  lithological  series. 
Above  the  Fernandan  system  is  an  Eparchean  group  of  rocks,  the 
stratigraphic  affinities  of  which  are  nearer  the  Archean  than  the 
Cambrian.  There  is  no  doubt  that  they  rest  unconformabl  v  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 
sandstones,  with  thinly  laminated  shales  and  chloritic  detrital  ma- 
terial ;  hard,  white,  laminated  quartz  rock  or  quartzite,  associated 
with  ferruginous  and  schist  layers;  ferruginous  shale  beds,  in  pari 
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  similar  lithological  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,  instead  of  blue.  The  Packsaddle  marbles  and  shaty 
beds  are  compared  with  the  Chuar;  the  Llano  quartzites  and  sand- 
stones, with  eruptives,  with  the  Grand  Canyon;  and  the  Mason  sandy 
shales  and  schists  with  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  \n\\>, 
or  by  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  Texan  system  are  never  involved  in  an  earlier  uplift  than  the 
north-south  trend. 
Comstock,80  in  1891,  further  describes  the  relations  of  the   pre 
Cambrian  rock  series  of  central  Texas.     The  Fernandian  system   is 
