MISCELLANEOUS  NONMETALS. 
VOLCANIC  ASH  NEAR  DURANGO,  COLO. 
By  Lester  II.  Woolsey 
Introduction. — Deposits  of  volcanic  ash  suitable  for  abrasive  material  have  been  reported 
to  occur  in  Colorado  north  of  La  Yet  a  Pass  in  Huerfano  County,  near  Wray  Station  in 
Yuma  County,  and  near  Durango  in  La  Plata  County.  In  this  paper  the  occurrence  near 
Durango  will  be  considered.  During  the  field  season  of  1905  the  writer  visited  the  deposits 
in  La  Plata  County  and  procured  the  data  for  the  following  sketch.  The  distribution, 
occurrence,  and  character  of  the  known  deposits  were  studied,  but  the  results  of  the  investi- 
{gation  were  such  that  no  careful  search  was  made  for  others. 
Distribution. — These  deposits  form  three  isolated  beds  which  lie  within  a  radius  of  4 
miles  of  Durango.  One  is  located  at  the  east  end  of  the  diy  valley  north  of  Animas  Sfe^r 
Mountain,  on  the  shoulder  of  the  southward-facing  spur.  Opposite  the  west  end  of  the^ 
same  valley,  on  the  crest  of  the  ridge  between  Dry  Gulch  and  Junction  Creek,  occurs 
another  bed.  Both  of  these  deposits  have  the  same  elevation — about  400  feet  above 
Junction  Creek  at  this  point.  The  third  bed  lies  nearly  east  of  Durango  on  the  east  slope 
of  Florida  Mesa,  250  feet  above  Florida  River.  All  these  beds  lie  at  nearly  the  same 
elevation  and  have  a  somewhat  similar  mode  of  occurrence. 
Occurrence-  The  broad  geologic  features  of  this  region  are  as  follows:  The  locks  consist 
mainly  of  the  conspicuously  colored  Jurassic,  Triassic,  and  Cretaceous  sediments  which 
overlie  the  Carboniferous  beds  exposed  farther  north.  The  whole  series  is  gently  upturned 
to  the  north  and  northwest  and  wraps  about  the  south  and  west  margin  of  an  old  Yrehcan 
surface.  Upon  the  beveled  edges  of  these  sediments  the  deposits  of  volcanic  ash  occur. 
The  deposit  north  of  Animas  City  Mountain  lies  across  the  edges  of  the  main  upper  sand- 
stone of  the  La  Plata  formation  and  the  overlying  thin  beds  (Jurassic-Triassic).  The 
actual  contact  is  concealed,  but  the  bottom  of  the  ash  is  very  Dear  bed  rock  and  20  feet 
may  be  considered  a  maximum  interval.  Above  the  ash  no  rocks  occur  in  place,  and  the 
covering  consists  of  rounded  pebbles,  possibly  of  glacial  origin,  dislodged  blocks  of  con- 
glomerates and  fragments  of  shale  belonging  to  the  Cretaceous.  The  form  of  the  deposit  is 
probably  that  of  a  horizontal  lens,  possibly  occupying  an  irregularity  in  the  bed-rock 
surface.  The  extent  of  the  lens  is  no  doubt  small,  for  ash  is  more  or  less  evident  about  100 
feet  horizontally  along  the  outcrop,  and  its  thickness  ranges  from  25  to  50  feet.  The 
weathered  exposure  shows  thin  horizontal  laminations,  as  if  the  material  were  deposited  in - 
quiet  water.  The  bed  on  the  ridge  southwest  of  Junction  Creek  presents  a  conspicuous 
outcrop  of  white  ash,  lying  upon  the  dark-gray  shale  of  the  Cretaceous.  It  is  covered 
chiefly  by  rounded  gravel,  probably  of  stream  origin,  mixed  with  shale  and  sandstone 
fragments.  In  size  and  form  the  bed  at  this  point  resembles  that  last  described.  The  lens 
has  been  partially  eroded  and  the  exposed  surface  is  not  over  150  feel  long  and  15  feet  wide, 
No  stratification  was  apparent  and  the  bed  exhibits  a  homogeneous  mass. 
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