402  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1905. 
the  surface  of  which  is  formed  by  the  "Dakota"  sandstone.  The  same  sandstone,  for  the 
most  part,  forms  the  crest  of  the  ridge  which  borders  the  canyon  on  the  west.  The  river 
has  cut  through  the  sandstone  and  underlying  formations  and,  in  the  south  part  of  the  area, 
has  cut  2,500  feet  into  the  crystalline  complex.  Near  the  north  edge  of  the  area  the  north- 
ward slope  carries  the  schists  below  drainage. 
Geology. — Overlying  the  schists  is  a  bed  of  massive,  fine-grained  sandstone  of  a  pinkish 
to  yellowish  or  salmon  color,  weathering  with  characteristic  rounded  outlines,  concave  and 
convex.  The  thickness  of  this  bed  in  the  southern  part  of  the  area  is  about  100  feet, 
thinning  to  the  northward  to  about  50  feet.  The  upper  part  becomes  flaggy,  then  shaly, 
and  passes  upward  into  the  gypsum  measures,  which  are  in  turn  succeeded,  either  directly 
or  with  an  interval  of  30  to  40  feet  of  shales  and  sandstones,  by  40  to  50  feet  of  blue  calca- 
reous shale,  with  interbedded  flaggy,  drab  limestones,  containing  a  few  fresh-water  fossils. 
This  is  followed  in  turn  by  400  feet  of  variegated,  but  predominantly  reddish  shales,  with] 
interbedded  red  and  buff  sandstones.  The  so-called  "Dakota"  comes  next  and  completes 
the  section  of  the  canyon  wall.  It  consists  at  the  bottom  of  from  40  to  60  feet  of  massive 
coarse-grained,  buff  sandstone,  with  sheets  and  strata  of  conglomerate,  the  pebble! 
averaging  one-half  inch  in  diameter  and  consisting  of  jaspers  and  cherts  of  various  colors. 
The  upper  member  of  the  formation  consists  of  similar  sandstones,  usually  not  so  thick, 
less  massive,  and  with  less  conglomerate.  Between  the  sandstone  members  is  a  series  of 
interbedded  dark  shales  and  buff  sandstones,  with  here  and  there  thin  beds  of  coal.  The 
thickness  of  the  whole  "  Dakota  "    series  is  about  200  feet. 
(rijpsum  measures. — With  an  average  thickness  of  110  feet  and  a  maximum  thickness  of 
150  feet,  gypsum  measures  outcrop  in  the  west  wall  of  Gunnison  Canyon  uninterruptedly 
from  a  point  below  the  mouth  of  Smiths  Fork  southward  to  Red  Rock  Canyon,  a  distance 
of  20  miles.  Immediately  west  of  Red  Rock  Canyon  they  cross  over  the  divide  between 
Gunnison  and  Uncompahgre  rivers  and  plunge  sharply  beneath  the  rocks  of  the  Uneom- 
pahgre  Valley.  From  this  point  southeastward  along  the  faulted  anticline  which  marks 
the  west  limit  of  Vernal  Mesa  the  gypsum  measures  are  either  wanting  or  concealed  by 
debris,  except  at  two  points  along  the  Tunnel  road  in  sec.  17,  T.  49  N.,  R.  7  W.,  where 
thin  beds  of  gypsum  appear 
The  stratigraphy  of  the  gypsum  measures  is  well  shown  in  the  following  sections,  taken 
at  different  points  along  the  west  wali  of  the  canyon: 
Sections  showing  gypsum  measures  in  Gunnison  Canyon,  Colorado. 
I. 
Feet. 
Shale  and  sandstone;  gypsum  in  gash  veins  and  lumps  constitutes  one-third 
of  the  whole 30 
Solid  granular  white  gypsum 10 
Solid  gypsum   with  some  shale  near  top 30 
Red  to  green  clay,  gypsiferous 14 
Solid  granular  gypsum 25 
Concealed,  shale  at  top 20 
Solid  white  gypsum 4 
II. 
Gypsiferous  sandstone 11 
Nearly  solid  gypsum,  shale  one-fifth  of  whole,  some  lumps  of  limestone oO 
Bluish  gypsiferous  shale ,  bands  and  gashes  of  gypsum 36 
Sandstone 6 
Solid  gypsum 4 
Arenaceous  gypsum 4 
Reddish  gypsiferous  shale 13 
Solid  gypsum 8 
III. 
Clay  and  gypsum;  gypsum  perhaps  one-third  of  whole 44 
Shale  and  arenaceous  gypsum 48 
White  gypsum  rock 5 
