744  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
valley  of  Honey  Creek,  L  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-Potsclam  age,  but  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  metamorphism  of  the  Llano  rocks.  No  rocks  of  undoubted 
Archean  age  were  observed. 
Shumard,83  in  1880,  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, 
Sacramento,  and  Horse  mountains  are  wholly  sedimentary.  None  of 
these  crystalline  rocks  are  regarded  as  pre-Cambrian. 
Glenn,84  in  1890,  describes  the  Azoic  rocks  as  consisting  princi- 
pally of  red  granite,  occasionally  gneissoid,  intersected  by  numer- 
ous 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  succeeding  the  granite.  Were  it  not  for  the  inter- 
position of  sandstone  between  the  granite  and  the  schists  they  would 
be  assigned  to  the  Azoic. 
Comstock,85  in  1890,  divides  the  Archean  rocks  of  central  Texas 
into  a  Burnetan  (  Laurentian?)  system  and  a  Fernandan  (Ontarian?) 
system. 
The  fundamental  gneisses  of  the  Burnetan  occupy  a  lens-shaped 
area  striking  N.  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  grade  on  the  one  hand  into 
quartzose  mica  schists  and  on  the  other  into  friable  sandy  gneisses 
and  fine-grained  binary  granites  and  graphic  granite.  Stratigraph- 
ically  the  group  is  divided  into  three  series;  these  are,  from  above 
downward:  (1)  Bodeville,  consisting  of  mica  schist  and  chlorite 
schists  (chiefly  acidic)  ;  (2)  Long  Mountain,  consisting  of  horn- 
blendic  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 
of  any  other  pre-Cambrian  system.  While  in  the  main  there  is  little 
difficulty  in  distinguishing  between  Fernandan  and  Burnetan  strata, 
metamorphism  has  caused  a  close  resemblance  in  many  exposures. 
The  general  succession  is,  from  above  downward:  Calcareous  rock, 
