266  PRE-CAMBRIAN   ROCKS   OE   NORTH   AMERICA.  [bull.  86. 
SECTION   III.      TEXAS. 
LITERATURE. 
EoEMER,22»in  1848,  mentions  granitic  rocks  at  several  points,  15  miles 
north  of  Fredericksburg,  on  the  banks  of  tlie  Llano,  in  the  country 
between  the  Llano  and  San  Saba,  and  between  the  Piedernaies  and  San 
Saba,  rivers.     These  granitic  rocks  are  surrounded  by  Paleozoic  strata. 
Shumard,23  in  I860,  describes  in  Burnet  county  rocks  upon  which 
rest  directly  the  fossiliferous  Potsdam. 
Shumard,24  in  1861,  describes  the  Primordial  rocks  of  Texas  as 
resting  upon  reddish  feldspathic  granite  very  similar  in  character  and 
composition  to  the  granites  of  Iron  mountain,  Missouri. 
Buckley,25  in  1866,  states  that  the  known  Azoic  rocks  of  the  state 
are  mostly  in  Llano  and  adjoining  counties.  There  are  here  granites 
with  steatite  or  soapstone,  immense  beds  of  iron  ore,  and  inetamorphic 
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  con- 
torted strata.  In  Mason  county  are  highly  inclined  micaceous  shales. 
At  Packsaddle  mountain  are  dark  shales  which,  near  Honey  creek,  ex- 
tend 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  and 
southwest  direction,  being  on  the  same  line  of  upheaval  as  the  Ozark 
mountains  of  Arkansas  and  the  Iron  mountains  of  Missouri. 
Buckley,26  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  underlie 
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  j 
metamorphic  rocks  associated  with  them.  Associated  with  the  granite 
in  Burnet  and  Llano  counties  are  immense  beds  of  magnetic  iron  ore. 
Buckley,27  in  1876,  describes  Azoic  granitic  rocks  in  many  of  the 
mountain  ranges  west  of  the  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  o 
these  rocks. 
Walcott,28  in  1884,  finds  that  the  Potsdam  sandstone  rests  uncon- 
formably on  a  great  formation  to  which  the  term  Llano  group  is  applied. 
