ISOLATED   AREAS    IN    MISSISSIPPI   VALLEY.  749 
sedimentary  rocks  of  the  Texas  area  have  been  correlated  by  Walcott 
and  Comstock  with  the  Algonkian  of  the  Grand  Canyon  region. 
J.  A.  Taff  and  E.  O.  Ulrich,  by  studies  made  in  1902  in  Llano  and 
San  Saba  counties,  Tex.  (results  unpublished),  found  that  the  Cam- 
bro-Ordovician  section  of  grits,  sandstone,  limestone,  and  dolomite 
is  identical  with  the  Cambro-Ordoviciari  section  in  the  Arbuckle  and 
Wichita  mountains  of  Indian  Territory  and  Oklahoma,  and  that 
the  Cambrian,  probably  "Potsdam"  of  Walcott,  rests  on  the  eroded 
surface  of  granite  cut  by  basic  dikes.  These  granites  probably  belong 
to  the  intrusives  in  Walcott's  Llano  series. 
Rocks  of  ancient  age  also  appear  in  western  Texas.  According 
to  G.  B.  Richardson  (see  summary,  pp.  746-748),  in  El  Paso  County 
there  are  two  areas  of  probable  pre-Cambrian  rocks.  One  is  in  the 
Franklin  Mountains  and  the  other  is  south  of  the  Sierra  Diablo 
adjacent  to  the  Texas  and  Pacific  Railway.  In  the  Franklin  Moun- 
tains the  pre-Cambrian  rocks  consist  of  a  monoclinal  succession  of 
quartzite  overlain  by  rhyolite,  intruded  by  diabase  and  granite,  and 
overlain  unconformably  by  Cambrian.  South  of  the  Sierra  Diablo 
is  a  complex  of  red  sandstone,  chert}^  limestone,  breccia,  igneous  in- 
trusions, and  schistose  rocks,  all  overlain  unconformably  by  Paleo- 
zoic strata. 
The  rocks  of  these  two  areas  are  tentatively  considered  to  be 
equivalent  to  the  Llano  series  of  central  Texas. 
NOTES. 
1  Local  details  of  geological  sections  on  the  St.  Peters,  Wisconsin,  Mississippi, 
Baraboo,  Snake,  and  Kettle  rivers,  by  B.  F.  Shumard.  Report  of  a  geological 
survey  of  Wisconsin,  Iowa,  and  Minnesota,  L852,  pp.  475-522. 
2  On  southern  Wisconsin,  including  the  iron,  lead,  and  zinc  districts,  with  an 
account  of  the  metamorphic  and  primitive  rocks,  by  James  G.  Percival.  Ann. 
Kept.  Geol.  Survey  Wisconsin,  1856,  pp.  111. 
3  The  iron  ores  of  Wisconsin,  by  Edward  Daniels.  Ann.  Kept.  Geol.  Survey 
Wisconsin  for  1857,  pp.  C>2. 
4 Physical  geography  and  general  geology,  by  James  Hall.  Kept.  Geol.  Sur- 
vey Wisconsin,  vol.  1.  1862,  p.  11. 
5  On  the  age  of  the  quartzites,  schists,  and  conglomerates  of  Sauk  County, 
Wis.,  by  B.  I  >.  [rving.     Am.  Jour.  Sci.,  3d  ser.,  vol.  :;.  L872,  pp.  !>::  99. 
'Report  on  the  geology  of  the  region  about  Devils  Lake,  by  James  II.  Eaton. 
Trans.  Wisconsin  Acad.  Sci.,  vol.  1.  1872,  pp.  124  L28.  Sec  also  On  the  rela- 
tions of  the  sandstone  conglomerates  and  limestone  of  Sauk  County.  Wis.,  to 
each  other  and  to  the  Azoic  quartzites.  Am.  Jour.  Sci..  3d  ser.,  vol.  5,  1st.".,  pp. 
444-447;  Trans.  Wisconsin  Acad.  Sci.,  v., I.  2,   1ST  I.  pp.   123   1l*T. 
7  Report  on  the  geological  survey  of  ilie  mineral  regions,  by  John  Murrish. 
Trans.  Wisconsin  Agr.  Soc,  1872  7."..  pp.    h'.'.i    194. 
s  Some  evidences  bearing  upon  the  methods  of  upheaval  of  the  quartzites  of 
Sauk  and  Columbia  counties,  by  T.  C.  Chamberlin.  Trans.  Wisconsin  A.cad. 
Sci.,  vol.  2,  1873,  pp.  129  1  :;•_'. 
9  On  fluctuations  in  level  of  the  quartzites  oi  Sauk  and  Columbia  counties,  by 
T.  C.  Chamberlin.     Idem,  pp.  133   138. 
