266         CONTRIBUTIONS    TO   ECONOMIC   GEOLOGY,    1907,   PART   II. 
Sections  of  coal  beds  in  the  Black  Rock  coal  group — Continued. 
Location. 
Section  of  coal  bed. 
Location. 
Section  of  coal  bed. 
Sec. 
T. 
R. 
Sec.                 T. 
R. 
31 
18 
100 
Ft.  in. 
Coal 2    3 
Shale 1    8 
Coal 8 
Shale 4 
Coal 6    5 
SE.iSE   \. 
13     23 
104 
Ft.  in. 
Shale 2 
Coal 7  10 
NE.iNE.L 
9  10 
SE.  JNE.  i. 
104 
11     4 
Shale 5 
Coal 10 
Shale 2 
Coal S 
Shale. 
SE.iSE.i. 
11 
18 
100 
Shale 3 
Coal 2    6 
24     23 
2    9 
14 
STRUCTURE. 
GENERAL    STATEMENT. 
The  structure  of  the  Rock  Springs  field  is  simple.  It  consists  of  a 
huge  dome  of  Cretaceous  and  Tertiary  rocks,  which  rises  in  the  midst 
of  the  nearly  horizontal  rocks  of  the  Red  Desert.  The  major  axis  is 
approximately  90  miles  long  and  trends  nearly  due  north  and  south 
close  to  the  west  limb  of  the  dome.  The  beds  along  the  west  limb 
dip  from  15°  to  30°  W.;  those  along  the  east  limb  dip  from  5°  to  10° 
E.  The  minor  axis  is  approximately  40  miles  long,  extending  across 
the  dome  in  an  east-west  direction,  passing  through  Rock  Springs 
and  a  point  3  miles  north  of  Black  Buttes.  Several  small  anticlines 
and  synclines  are  developed  upon  the  main  dome,  but  for  the  most 
part  they  are  small  and  unimportant.  The  major  anticlinal  axis 
plunges  slightly  in  going  northward  and  southward  from  the  central 
portion  of  the  dome,  and  finally  loses  itself  in  the  Tertiary  beds. 
The  oldest  beds  composing  this  dome  are  exposed  in  the  vicinity  of 
Baxter  station,  on  the  Union  Pacific  Railroad. 
STRUCTURAL    RELATIONS    OF    COAL    GROUPS. 
The  coal  groups  in  this  area  lie  around  the  central  portion  of  the 
dome  in  four  zones,  of  which  three  are  continuously  exposed.  For 
convenience  in  this  discussion  these  zones  are  given  the  following 
names,  beginning  with  the  lowest  and  continuing  upward:  Rock 
Springs  zone,  Almond  zone,  Black  Buttes  zone,  and  Black  Rock 
zone.  The  structural  relations  of  the  two  lower  coal  groups,  Rock 
Springs  and  Almond,  are  those  of  two  conformable  series  that  occupy 
concentric  oblong  belts  around  the  central  portion  of  the  dome, 
which  is  composed  of  rocks  that  are  not  coal  bearing.  These  two 
coal  zones  are  separated  from  each  other  by  a  belt  of  barren  sand- 
stone, whose  stratigraphic  thickness  is  about  800  feet. 
