748  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
fragments  of  fine  red  sandstone  and  cherty  limestone.  In  places 
these  rocks  have  been  cut  by  both  acidic  and  basic  dikes. 
The  schistose  area  south  of  the  Texas  and  Pacific  Railway  is  com- 
posed of  altered  siliceous  strata  consisting  of  quartz  schists,  quartzites, 
and  clay  slates,  together  with  subordinate  basic  igneous  rocks  which 
have  been  altered  to  chlorite  schists.  These  rocks  outcrop  in  an  area 
of  dissected  ridges  and  intervening  valleys  which  trend  northeast- 
southwest.  The  ridges  are  underlain  by  hard  rocks  and  the  valleys 
by  softer  ones,  the  topographic  trend  corresponding  with  the  strike 
of  the  rocks.  The  dip  is  steep  southeastward,  varying  between  25° 
and  85°. 
The  relations  of  the  rocks  in  the  two  areas  north  and  south  of  the 
railroad  are  concealed  by  Quaternary  wash  deposits.  In  both  areas 
the  tilted  ancient  rocks  are  overlain  by  almost  flat-lying  Paleozoic 
strata  which  contain  a  basal  conglomerate  composed  of  pebbles  of  the 
underlying  rocks.  The  lowest  of  these  is  a  coarse  red  sandstone 
averaging  about  400  feet  in  thickness,  of  probable  Cambrian  age,  but 
in  which  no  fossils  other  than  annelid  borings  have  been  found.  This 
sandstone  is  overlain  by  a  thick  mass  of  limestone  that  contains  abun- 
dant fossils,  determined  by  Ulrich  to  be  of  lower  Ordovician  (Beek- 
mantown)  age. 
In  both  the  Franklin  Mountains  and  Sierra  Diablo  areas  the  pre- 
Cambrian  (?)  rocks  are  in  large  part  of  sedimentary  origin,  although, 
besides  the  fact  that  quartzites  are  present  in  both  areas,  there  is  little 
lithologic  similarity  in  the  deposits.  The  origin  and  stratigraphic 
relations  of  these  trans-Pecos  rocks  suggest  that  they  are  probably 
equivalent  to  the  Llano  series  of  central  Texas. 
SUMMARY   OF  PRESENT   KNOWLEDGE. 
In  central  Texas  is  a  series  of  sedimentary  rocks,  named  Llano  by 
Walcott,  which  consists  of  alternating  beds  of  shales,  slates,  sand- 
stones, quartzites,  limestones,  ferruginous  rocks,  carbonaceous  or 
graphitic  schists,  mica  schists,  and  chlorite  schists.  Comstock  divides 
the  sedimentary  rocks  into  two  "  systems,"  between  which  he  believes 
there  is  an  unconformity.  There  also  occur  in  this  area  granites  and 
gneisses,  a  part  of  which  are  placed  as  a  separate  "  system  "  by  Com- 
stock, but  which  are  regarded  by  Walcott  as  intrusive  in  the  sedi- 
mentary rocks.  Whether  all  of  the  granites  or  gneisses  are  intrusive 
or  not,  it  is  agreed  by  both  that  the  clastic  series  are  cut  by  numerous 
eruptives,  both  basic  and  acidic,  of  which  granite  is  the  most  prom- 
inent. Kesting  unconformably  upon  the  deeply  eroded  pre-Cambrian 
are  the  Cambrian  ("Potsdam")  rocks.  It  is  clear  that  in  central 
Texas  there  is  one,  and  possibly  two,  series  of  Algonkian  rocks,  but 
whether  the  Archean  is  also  represented  is  as  yet  undetermined.    The 
