vanhiss.j       ISOLATED    AREAS    OF    THE    MISSISSIPPI    VALLEY.  2G7 
These  rocks  are  alternating  beds  of  sandy  shales,  sandstones,  lime- 
stones, and  schists*  that  have  a  dip  from  15°  to  40°.  They  arc  little 
metamorphosed.  The  overlying  sandstone  in  its  fossils  is  like  the 
Tonto  group  of  the  Grand  canyon,  and  the  Llano  group  is  correlated 
with  the  Grand  canyon  and  Chuar  series  of  the  Grand  canyon  on  the 
basis  of  position  and  lithological  character.  The  best  exposures  are 
at  Packsaddle  mountain,  in  Llano  county,  where  the  horizontal  Pots- 
dam rests  on  the  uptilted  and  eroded  Llano  beds.  Across  the  valley 
of  Honey  creek,  4  miles  west  of  Packsaddle  mountain,  the  strata  of 
the  Llano  group  have  been  more  metamorphosed,  plicated,  and  broken 
by  intrusive  dikes  of  granite.  The  intrusive  rocks  are  of  pre-Potsdam 
age,  Jbut  largely  the  result  of  extrusion  of  granite  at  or  near  the  close 
of  the  erosion  of  the  Llano.  They  are  the  chief  cause  of  the  metamor- 
phism  of  the  Llano  rocks.  No  rocks  of  undoubted  Archean  age  were 
observed. 
Shumard,29  in  188G,  describes  as  eruptive  rocks  the  granites,  por- 
phyries and  basic  rocks  which  compose  the  whole  of  Wichita,  Limpea, 
Hueco,  and  Mimbres  mountains.  In  the  Organ  mountains  are  partly 
sedimentary  and  partly  eruptive  rocks;  while  the  Guadalupe,  Sacra- 
mento, and  Horse  mountains  are  wholly  sedimentary.  None  of  these 
crystalline  rocks  are  regarded  as  pre-Cambrian. 
Glenn30,  in  1890,  describes  the  Azoic  rocks  as  consisting  principally 
of  red  granite,  occasionally  gneissoid,  intersected  by  numerous  nearly 
vertical  dikes  of  quartz  rock.  West  of  the  granite  in  Llano  county  is 
an  extensive  field  of  schist,  sandstone,  and  limestone  of  uncertain  age. 
At  Spring  creek,  in  Burnet  county,  is  also  a  small  schist  formation  suc- 
ceeding the  granite.  Were  it  not  for  the  interposition  of  sandstone 
between  the  granite  and  the  schists  they  would  be  assigned  to  the 
Azoic. 
Comstock:;1,  iii  1890,  divides  the  Archean  rocks  of  central  Texas  into 
a  Burnetan  (Lauren tian?)  system  and  Fernandan  (Ontarian4?)  system. 
The  fundamental  gneisses  of  the  Burnetan  occupy  a  lens-shaped  area 
striking  1ST.  75°  W.,  and  they  are  well  exposed  in  Burnet  county.  Within 
the  group  there  are  no  unconformities.  The  rocks  of  the  system  are 
largely  gneisses,  but  they  graduate  upon  the  one  hand  into  quartzose 
mica-schists,  and  upon  the  other  into  friable  sandy  gneisses  and  fine- 
grained binary  granites  and  graphic  granite.  Stratigraphically  the 
group  is  divided  into  three  series  from  above  downward,  (1)  B.)deville, 
consisting  of  mica-schist  and  chlorite-schists  (chiefly  acidic);  (2)  Long 
mountain,  consisting  of  hornblendic  and  pyroxenic  rocks  (basic);  (3) 
Lone  grove,  consisting  of  gneiss,  granite,. etc.  These  rocks  are  compared 
with  Lawson's  lake  of  the  Woods  Archean.  The  igneous  eruptions  of 
the  Burnetan  are  of  different  ages,  some  of  them  earlier  and  some  later 
than  the  Potsdam. 
The  Fernandan  or  Ontarian  system  is  well  exposed  along  the  valley 
of  San  Fernando  creek.     Its  exposures  are  more  extensive  than  those 
