VOLCANIC    ASH    NEAR    DURANGO,    COLO. 
477 
The  occurrence  on  Florida  Mesa  was  at  the  time  of  visit  not  well  exposed,  but  over  an 
area  approximating  2  acres  fragments  of  ash  were  thrown  out  by  ground  animals.  It  was 
reported  to  have  been  uncovered  in  several  places  years  ago  and  excavated  to  a  depth  of  3 
feet.  The  covering  consists  of  a  few  feet  of  soil  and  mesa  gravel,  and  the  underlying  bed 
rock  belongs  to  the  coal-bearing  series  of  the  Cretaceous.  From  the  above  facts  concerning 
these  ash  beds  it  may  be  concluded  that  they  occur  with  no  regard  to  horizon  in  the  sedi- 
mentary series,  that  they  are  all  overlain  by  surficial  deposits,  and  that  they  are  small 
in  extent,  probably  occupying  basins  in  an  old  topography.  This  old  land  surface,  if  we 
may  judge  from  the  elevation  of  the  ash  deposits,  was  not  so  deeply  dissected  by  streams 
as  it  is  to-day,  and  the  present  dissection  may  be  considered  as  deepest  in  the  lower  courses 
of  the  present  streams.  New  deposits  of  volcanic  ash,  therefore,  are  not  likely  to  be  found 
along  the  lower  courses  of  streams,  but  may  be  discovered  on  the  crests  of  ridges  or  high 
up  on  the  valley  sides. 
Character. — All  three  occurrences  are  very  similar  in  appearance  and  exhibit  a  very  fine 
dust-like  powder  of  white  opaque  flakes  which  glisten  in  the  sunlight.  The  powder  has  a 
gritty  feel  and  was  not  seen  consolidated  in  a  hard  mass.  Under  the  microscope  the 
powder  is  seen  to  consist  chiefly  of  thin  minute  flakes  with  angular  edges  and  fair  uniformity 
in  size  for  all  the  beds.  Nearly  all  the  flakes  exhibit  the  transparent  amorphous  and 
isotropic  character  of  glass,  but  some  particles  show  interference  colors,  extinction,  and 
other  characteristics  of  quartz  grains.  Few  of  the  glass  flakes  show  air  bubbles,  and  the 
material  on  the  whole  resembles  powdered  volcanic  pumice  and  may  be  considered  volcanic 
dust  or  ash.  It  is  entirely  similar  to  the  volcanic  ashes  from  Montana  and  Idaho  collected 
by  Peale  of  the  Hayden  Survey  and  later  described  by  Merrill.  The  chemical  content  is 
no  doubt  largely  silica,  and  the  composition  is  probably  close  to  that  of  similar  ashes 
already  analyzed,  as  follows: 
Analyses  of  volcanic  ash. 
1. 
2. 
Si02 - 
68.92 
16.22 
Trace. 
1.62 
1.56 
4.00 
68.91 
AI2O3+  Fe203 
6.12 
MgO 
CaO 
3.44 
Na20 
3.09 
K20 
.36 
S03 • 
8.88 
a  6.  00 
1.60 
8.75 
Water                                                                
99.92 
99.55 
a  Organic. 
G.  P.  Merrill,  Am.  Jour.  Sci 
vol.  32,  1886,  p.  202;  J.  E.  Whitfield, 
1.  Marsh  Creek  Valley,  Idaho, 
analyst. 
2.  Average  sample  from  Nebraska.     E.  H.  Barbour,  Proc.  Nebr.  Acad.  Sci.,  No.  5,  1895,  p.  13;   H.  II. 
Nicholson,  analyst. 
Relations. — Deposits  of  this  sort,  to  judge  from  published  reports,  are  very  common  through 
Nebraska,  reaching  into  South  Dakota  and  Wyoming  on  the  north  and  west  and  into  Kansas 
and  Oklahoma  on  the  south.  In  many  localities  the  beds  are  apparently  very  extensive 
and  in  some  places  very  thick,  but  usually  they  are  thinner  than  the  Durango  beds.  The 
bulk  of  the  deposits  on  the  Great  Plains,  as  stated  by  Barbour,  are  fine  and  uniform,  but 
variations  are  known,  and  the  ash  may  be  coarse,  of  a  dark-gray  color,  or  mixed  with  sand, 
silt,  etc.  He  notes  the  eastern  limit  to  be  Iowa,  where  the  deposits  are  very  fine,  while  in 
Bull.  285—06—31 
