384  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1903.  [bull.  225. 
Here  and  there  among  the  fields  or  in  the  side  of  an  irrigating  ditch, 
the  outcropping  of  a  thin  harder  bed  may  be  indicated  by  a  few  frag- 
Fig.  10.— Sketch  map  of  the  Boulder  oil  field,  Colorado. 
ments  and  a  sandier  soil,  but  generally  the  bed  is  not  strong  enough 
to  cause  even  a  small  ridge.     A  little  excavation  may  sometimes  reveal 
