338         CONTRIBUTIONS   TO   ECONOMIC   GEOLOGY,   190*7,   PART   II. 
GEOLOGY. 
STRATIGRAPHY. 
GENERAL    SECTION. 
The  formations  known  to  occur  within  the  area  included  in  this 
report  are  as  follows: 
Generalized  section-  of  rocks  in  coal  field  between  Gallina  and  Raton  Spring,  N.  Mex. 
System. 
Series  or  formation. 
Thickness 
in  feet. 
s 
0-50 
(?) 1,000 
Unconformity. 
110 
Unconformity  (?). 
690 
(Unconformity. 
Laramie  formation 
900 
250-2,000 
500-1,000 
[Dakota  sandstone. 
CRETACEOUS  ROCKS. 
Dakota  sandstone. — The  Dakota,  or  basal  formation  of  the  Creta- 
ceous throughout  the  San  Juan  region,  is  exposed  in  a  prominent  hog- 
back along  the  west  slope  of  the  Sierra  Nacimiento.  It  consists 
chiefly  of  hard  quart zite  with  intercalated  shale  toward  the  top,  but 
near  the  base  is  made  up  of  reddish  sandstone  and  yellowish  shale  that 
grade  gradually  into  the  Jurassic-Triassic  rocks  below.  No  signs  of 
coal  beds  were  observed  in  the  Dakota,  but  they  are  known  to  occur  in 
other  parts  of  the  basin. 
Mancos  shale. — The  Mancos  shale  rests  conformably  upon  the 
Dakota.  In  the  type  locality  at  Mancos,  Colo.,  the  formation  is  all 
shale,  but  in  New  Mexico  there  are  transitional  sandy  beds  toward  the 
top.  At  a  point  10  miles  north  of  Gallina  a  part  of  the  formation 
becomes  arenaceous  and  forms  a  hogback  in  the  shale  valley.  This 
sandy  bed  is  about  30  feet  thick  and  about  275  feet  below  the  top  of 
the  formation.  It  is  no  doubt  the  beginning  of  the  sandstone  and 
shale  formation  that  increases  in  thickness  toward  the  south  and  is 
coal  bearing  on  the  south  side  of  the  basin. a  The  siliceous  phase  is 
continuous  along  the  foot  of  the  Sierra  Nacimiento,  but  is  prominent 
only  south  of  Cuba.  In  the  vicinity  of  San  Miguel,  12  miles  down 
Rio  Puerco  from  Cuba,  the  writer  observed  a  thickness  of  300  feet  of 
argillaceous  sandstone  and  sandy  shale  in  the  Mancos,  grading  upward 
through  a  transition  zone  to  the  Mesaverde. 
Mesaverde  formation. — The  Mesaverde  is  the  most  important  coal- 
bearing  formation  in  the  area,  as  well  as  throughout  the  San  Juan 
a  See  reports  of  Schrader  and  Shaler,  already  cited. 
