ISOLATED   AREAS   IN    MISSISSIPPI   VALLEY. 
Buckley,79  in  1866,  states  that  the  known  Azoic  rocks  of  Texas  are 
mostly  in  Llano  and  adjoining*  counties.  There  are  here  granites  with 
steatite  or  soapstone,  immense  beds  of  iron  ore,  and  metamorphic 
rocks,  consisting  chiefly  of  slates,  mica  schist,  and  gneiss  with  quartz 
veins.  The  granites  of  Burnet  County  probably  belong  to  a  later 
period  of  elevation  than  the  Azoic.  Here  the  metamorphic  rocks  are 
on  the  outskirts  of  the  granite,  in  nearly  vertical,  more  or  less  broken 
or  contorted  strata.  In  Mason  County  are  highly  inclined  micaceous 
shales.  At  Packsaddle  Mountain  are  dark  shales  which,  near  Honey 
Creek,  extend  unconformably  beneath  the  nearly  horizontal  layers  of 
Potsdam  sandstones  and  limestones.  In  Mason  County  is  a  very 
large  deposit  of  iron  ore,  which  is  believed  to  be  a  true  vein.  Another 
bed  of  iron  ore  lies  between  two  granite  ridges  and  is  traversed  by 
veins  of  quartz.  House  Mountain,  consisting  of  granite,  is  capped  by 
massive  beds  of  nearly  horizontal  sandstone.  The  Azoic  rocks  trend 
in  a  northeast-southwest  direction,  being  on  the  same  line  of  upheaval 
as  the  Ozark  Mountains  of  Arkansas  and  the  Iron  Mountains  of 
Missouri. 
Buckley,80  in  1874,  describes  as  resting  unconformably  beneath  the 
Potsdam,  in  Llano  County,  shales  and  argillites  which  lithologically 
resemble  the  old  slates  of  Vermont  and  New  Hampshire.  They  are 
barren  of  fossils.  Locally  a  slaty  cleavage  is  developed.  Sometimes 
the  slate  is  changed  into  a  gneissoid  rock,  all  gradations  of  the  change 
being  seen.  Friable  mica  slates  containing  garnet  sometimes  under- 
lie the  granite.  These  rocks  are  referred  to  the  Laurentian.  Most, 
and  probably  all,  of  the  granites  of  this  region  are  of  a  later  period 
than  the  metamorphic  rocks  associated  with  them.  Associated  with 
the  granite  in  Burnet  and  Llano  counties  are  immense  beds  of  mag- 
netic iron  ore. 
Buckley,81  in  1876,  describes  Azoic  granitic  rocks  in  many  of  the 
mountain  ranges  west  of  Pecos  River.  At  a  number  of  places  basal- 
tic rocks  occur.  All  the  igneous  rocks  north  of  the  Pecos  are  either 
of  upper  Cretaceous  or  Tertiary  age,  as  is  shown  by  the  uptilted  strata 
of  these  rocks. 
Walcott,82  in  1884,  finds  that  the  Potsdam  sandstone  rests  uncon- 
formably on  a  great  formation  to  which  the  term  Llano  group  is  ap- 
plied. These  rocks  are  alternating  beds  of  sandy  shales,  sandstones, 
limestones,  and  schists,  that  have  a  dip  of  15°  to  40°.  They  are  Little 
metamorphosed.  The  overlying  sandstone  in  its  fossil-  i-  like  the 
Tonto  group  of  the  Grand  Canyon,  and  the  Llano  group  is  correlated 
with  the  Grand  Canyon  and  Chnar  series  of  the  Grand  Canyon  on 
the  basis  of  position  and  lithological  character.  The  besl  exposures 
are  at  Packsaddle  Mountain,  in  Llano  County,  where  the  horizontal 
Potsdam  rests  on  the  uptilted  and  eroded  Llano  In^U.     Across  the 
