vanhise.i       ISOLATED    AREAS    OF    THE    MISSISSIPPI    VALLEY.  2G0 
Comstock,32  in  1891,  further  describes  the  relations  of  the  pre-Cam- 
brian  rock  series  of  central  Texas.  The  Fernandian  system  is  hcl<l  to 
rest  unconformably  upon  the  Burnetian,  because  no  other  terrane 
within  the  Burnetian  has  structural  planes  or  breaks  following  a  course 
N.  75°  W.,  while  every  other  axis  of  uplift  is  traceable  through  the 
rocks  of  the  Burnetian  system,  and  because  the  basal  members  of  the 
Fernandian  system  are  made  up  in  part  of  material  apparently  derived 
from  the  Burnetian  rocks.  That  the  Texian  (Algonkian!)  system  rests 
unconformably  upon  the  Fernandian  is  concluded  from  facts  of  the 
same  character  as  those  which  show  the  discordance  between  the  Burne- 
tian and  Fernandian.  The  nearly  due  north-south  strikes  of  these 
rocks  are  commonly  peculiar  to  them,  the  earlier  fractures  and  lines  of 
uplift  being  invariably  absent,  but  the  later  ones  can  be  more  or  less 
distinctly  traced  through  the  members  o'f  this  system.  There  are  locali- 
ties exhibiting  the  juxtaposition  of  the  Texian  with  the  underlying 
Fernandian,  in  which  the  nonconformability  between  the  two  is  seen. 
These  relations  are  seen  south  and  southeast  of  Packsaddle  mountain, 
southwest  of  Sharp  mountain,  in  portions  of  the  country  north  of  Lock- 
hart  mountain,  north  and  northeast  of  Mason,  in  the  Beaver  creek  val- 
ley, and  elsewhere  in  Mason  county.  Further,  the  derivative  character 
of  the  Texian  beds  is  a  marked  feature.  In  the  Fernandian  is  a  great 
development  of  magnetites.  While  these  deposits  appear  to  be  in  dis- 
continuous lenses  or  bosses  across  the  region,  there  is  almost  always  an 
indication  of  continuity  in  the  shape  of  a  line  of  ferruginous  soil  or 
other  landmark.  The  iron  deposits  have  above  them  carbonaceous  and 
calcareous  beds  and  below  them  quartzose  beds. 
SUMMARY   OF    RESULTS. 
It  appears  that  in  central  Texas  there  are  two  thick  series,  of  clastic 
origin  which  are  of  pre-Gambrian  age.  The  upper  is  Walcott's  Llano  or 
Comstock's  Texian,  and  the  lower  Coinstock's  Fernandian.  The  first 
of  these  is  but  little  altered,  the  second  is  considerably  metamorphosed 
and  has  associated  with  it  a  greater  quantity  of  eruptive  rocks.  Be- 
tween these  series  it  is  asserted  by  Comstock  that  there  is  a  great  un- 
conformity, as  shown  by  numerous  contacts,  by  Fernandian  debris  in 
the  Texian,  and  by  the  fact  that  the  Texian  rocks  are  never  involved 
in  the  earlier  uplifts  which  have  affected  the  Fernandian. 
The  Archean  is  represented  by  the  Burnetian.  Between  the  Burne- 
tian and  the  Fernandian  an  unconformity  is  maintained  by  Comstock 
upon  essentially  the  same  grounds  as  between  the  Texian  and  Fernan- 
dian; that  is,  there  are  unconformable  contacts  between  the  series;  the 
Fernandian  bears  debris  from  the  Burnetian,  and  the  Burnetian  has 
beet)  affected  by  orographic  movements  which  are  earlier  than  the  Fer- 
nandian. 
